List of The Big Bang Theory episodes

Sheldon and Leonard go over to Penny's apartment to drop off a package that came for her. Sheldon is deeply disturbed when he sees how messy and disorganized her apartment appears. Later that night, when Leonard is sleeping, Sheldon sneaks into Penny's apartment to clean and organize it. Leonard catches him and tries to stop him, but Sheldon is obsessed. The next morning, Penny yells at them, prompting Leonard to ask how she knew (Sheldon had left an organizational diagram for her closet). Sheldon goes to apologize but Penny can not believe his attitude and remains upset. Later on Leonard goes to apologize, prompting Penny to hug him and say 'we're okay'.

When Leonard sees Penny kissing a guy in front of her apartment door, he is devastated thinking she has 'rejected' him. The guys convince him to ask someone out at work, but when he tries and is brutally turned down by a geeky girl scientist named Leslie Winkle (Sara Gilbert), he is even more depressed. As Leonard slips deeper into his depression (which involves thinking about buying a cat and listening to emo music), Sheldon (in an attempt to not have a cat) points out that he was not actually rejected, as he had not asked Penny out. Leonard agrees and promptly asks Penny out to dinner, but she mistakes his invite as an invitation to hang with all the guys. Despite being aware of this, Leonard keeps the date and makes up excuses as to why the guys were absent.

On the date, Penny mentions the man Leonard had seen was not her boyfriend, and that she has a habit of having weekend long flings to get over relationships. Excited, Leonard begins to become more confident but, after accidentally hurting himself, the date becomes a disaster and Penny takes him home. Leonard does not get the courage to reveal his true feelings to Penny, even though she had suspected what he was up to.

Sheldon is fired from his job as a physicist at the university when he insults his (in his opinion) less-gifted new boss Dr. Eric Gablehauser (Mark Harelik). His change of circumstance sends him into a downward spiral of depression in which Leonard tries to cure by sending for Sheldon's mom (Laurie Metcalf). When she arrives for a visit, the boys realize she is completely the opposite from what they expected. She is down-to-earth, sweet, and a devout Christian ("All that science stuff that comes from Jesus"). After waiting it out his mother finally forces Sheldon to apologize and he is given his job back, after she flirts with Dr. Gablehauser (she is apparently widowed). Sheldon asks if Dr. Gablehauser will be his new Daddy and his mother replies, "We'll see..."

While eating at The Cheesecake Factory where Penny works, the guys run into Leslie Winkle (Sara Gilbert), a fellow scientist who works with Leonard (and previously turned him down for a date saying she had no attraction to him). She suggests that Leonard play in her string quartet as they are in need of a new cellist. However what she really wants is to get personal with Leonard.

When Sheldon comes home and finds a necktie on Leonard's door, he runs to Penny’s apartment to ask her what the necktie means. Penny tells him and seems slightly disappointed, though she does congratulate Leonard later on. Leonard tries to figure out what her words meant, but eventually gives up, deciding he should choose a real relationship (Leslie) over an imaginary one (Penny). However when he arrives in the lab, Leslie makes it clear she used him just to satisfy her sex drive, saying she's 'Good till New Year's'. Leonard later tells Penny, who walks away with a smile on her face; she seems to be quite happy to hear this news.

The guys are invited to Penny's Halloween party, first turning it down as they "don't dance" but changing their mind when costumes are mentioned. Once at the party, only Wolowitz and Leonard attempt to socialize - with mixed success. Penny's ex-boyfriend Kurt shows up and humiliates Leonard, causing him to leave. Penny goes to check on him and apologizes. Both drunk and upset, she kisses Leonard, which he refutes with the obvious reasons as to why she's doing it. Penny says he's sweet and wonders why all men can't be like him, with Leonard replying, "Because the human race wouldn't survive". Penny goes to leave and kisses him once more in the doorway. Meanwhile, Wolowitz can not find Koothrappali, who ends up going home and has sex with a girl he had met at the party, who found him to be a 'good listener' (as Koothrappali is so shy he can not speak to attractive women).

Penny's slutty 'friend' Chrissy (Brooke D'Orsay) from Nebraska is in town, and ends up being seduced by Wolowitz. The pair take over her apartment causing her to sleep on Leonard and Sheldon's couch for the night. With Wolowitz gone, Penny takes his place on their Halo team, causing Sheldon much distress as she is naturally good at it. Chrissy moves in with Wolowitz (and his mother) taking up all his time. Sheldon is distraught over the odd number in the group believing they can no longer function. He's even more distraught when he relents and lets Penny join in on Halo night, but she declines the offer to instead go dancing with her friend. Believing they are falling apart as a group, the guys go to speak to Wolowitz. When they arrive they hear Chrissy and Wolowitz's mother arguing loudly, causing Chrissy to leave. Wolowitz, once again single, says 'So, Halo night?' and the group is restored.

Koothrappali is overwhelmed after his parents arrange a blind date with a girl from his childhood, as he can not talk to attractive members of the opposite sex. Penny takes up bar tending for some extra money; and as she practices mixing drinks with the guys they discover that after a few drinks Koothrappali can actually talk to women. However on his date at Penny's restaurant (with all the guys watching from the bar), he becomes obnoxious, and the girl ends up leaving with Sheldon after he accidentally complimented her by comparing her to a beautiful Indian fairy tale princess.

Koothrappali and Leonard return to Leonard's apartment where Koothrappli tries to explain to his parents what happened. Even though he is already upset things become worse when his parents find out he was drinking and he gets lectured and has to leave the apartment. Sheldon returns and Leonard asks him what happened after the pair had left the bar. Sheldon seems to not have thought of it as a date, implying he will not see her again (making Leonard wonder out loud, "I wonder who will tell his parents they're not getting grandchildren").

Cleaning up after an internet experiment, Leonard discovers a letter in the trash can. He and Sheldon had been invited to present some research they had done together at a physics conference. However Sheldon does not believe in such things and says if he won't go, no one can. Leonard defies him and says he will go and present the findings on his own. This causes a rift between the two which Penny tries to mend but accidentally makes worse. At the conference Sheldon appears and not only tries to "blow up" Leonard's head with his mind a la the movie Scanners, but has a physical altercation with Leonard. In the end, the fight is captured by Wolowitz and put on Youtube.

Penny was given a part in Rent, though after hearing her sing "Out Tonight" extremely off-key ("What the hell is that?" "I don't know, but if cats could sing...they'd hate it too.") Leonard lies to avoid seeing her performance so he won't hurt her feelings. Sheldon becomes paranoid that Leonard's lie was 'laughably transparent' and eventually creates an intricate lie about his non-existent drug addicted cousin Leo (DJ Qualls), who was in need of an intervention (and thus why they could not see the play). Sheldon becomes so obsessed he actually has a fellow physcist (who double majored in theatre) play the cousin to convince Penny that the lie is true. In the end, Leonard still has to see the tape of Penny's performance, thus the lie did him no good at all.

Penny returns from visiting family in Nebraska, and as she picks up her mail from Leonard, she mentions it was a bad trip because 'everyone got sick'. Sheldon freaks out as according to Leonard 'he's a bit of a germaphobe'. Sure enough, Sheldon becomes sick and instantly becomes the biggest pain possible. Leonard and the guys hide from him at a Planet of the Apes marathon, leaving Penny unwillingly to take care of Sheldon. However, Leonard accidentally breaks his glasses and must re-enter the apartment, where Penny catches him and sticks him with Sheldon. In the end, both Leonard and Sheldon are sitting sick and injured on the couch. Sheldon wonders if Penny will come take care of them, to which Leonard replies, "I don't think Penny is ever coming here again."

Sheldon is overcome with I.Q. envy when he meets a smarter 15-year-old science prodigy named Dennis Kim (Austin Lee). Losing faith in his own research, he annoys his circle of friends and colleagues by criticizing their work (prompting them to say "Go away!" whenever he's around). Deciding his work in physics is useless, he decides to aim for the Nobel Peace Prize by creating a 'Nuevo Jerusalem' for the Jewish people in the Mexican desert (thus creating peace in the Middle East).

The guys, tired of his insults and his craziness decide to take Dennis out of the picture by getting him distracted with girls. At his welcome party, their plan works (though mostly by accident) and Dennis leaves with a blonde 15 year old (Unfabulous's Emma Degerstedt) for the mall. Excited, Sheldon declares his work will continue on and 'human relationships will continue to baffle and repulse me'. The guys were excited Sheldon was back to his old self, but Leonard, Wolowitz and Rajesh were upset and perplexed at how Dennis was able to get girls yet they were not.

The guys decided to compete in a physics bowl after finding out that the previous competitors have dropped out to form a barbershop quartet and got a gig playing Knott's Berry Farm, but find Sheldon's showboating too much and kick him off the team, telling him "You're annoying and no one wants to play with you". They even buy him a Batman Cookie jar to soften the blow of being cut from the team. Upset, Sheldon vows to form his own team and compete against them. The guys in need of a 4th member ask Leslie Winkle to join them. After being told it is to defeat Sheldon, she joins as Sheldon had once told her that she should give up her research to bear children and do laundry.

At the bowl, the guys team is called PMS, for Perpetual Motion Squad ("We can go all night!"). Sheldon assembles the 3rd floor janitor, the lunch lady, and her son (or butcher) as he could not compete alone. Sheldon's team is named AA for Army Ants. PMS wins by one question, when Sheldon (unable to answer the question) won't let Dr. Gablehauser accept the janitor's answer, -8πα, even though it turns out to be correct. The janitor proceeds to explain that in America, he is a janitor, but in the former Soviet Union, he was a physicist. In the end, Penny declares she will tell who is officially the smartest by asking a bunch of pop culture questions. Despite having a "combined IQ of 360", neither of the boys is able to answer any of the questions.

This episode was originally titled The Nerdmabelia Scattering. Leonard buys a replica time machine from the 1960 film, The Time Machine, online believing it to be a miniature... though it turns out to be full-sized. As the boys move it into the apartment, they accidentally make Penny late for work causing her to lash out at Leonard for his nerdy ways. Upset, Leonard decides to get rid of all his 'nerdmabelia' and the guys try to convince him not to (or sell it to them, whichever). In the end, Sheldon confronts Penny about her own love of childish things (Hello Kitty, Beanie Babies, My Little Pony, etc.) and she apologizes to Leonard. However, she has a new boyfriend named Mike, and once Leonard finds this out, he reverts to his old ways.

The episode is sometimes called The Shiksa Indeterminacy. The guys see an attractive woman in Sheldon's office and can not believe their eyes. The woman turns out to be his twin sister Missy (Courtney Henggeler), who was in town for a wedding and needed Sheldon to sign some papers for her father's estate. The boys are smitten with her and Leonard insists she stay with them. The boys instantly try to hit on her, causing Penny to come to her rescue.

Meanwhile, Leonard becomes sneaky and convinces Sheldon to be protective of Missy (so he can have a chance at her over the other guys)...however, much to Leonard's chagrin, Sheldon forbids him to go out with his sister as well. Upon learning what Sheldon is up to, Missy has a heart-to-heart with Sheldon and, after getting crotch-punched by Missy, Sheldon agrees "Missy can date whoever she wants". After a Wii Boxing battle Leonard wins first chance, but Missy turns him down (something they had apparently not expected). Wolowitz tries next, but she turns him down as well.

Meanwhile, Koothrappali takes part in a drug trial, for a drug that can cure his extreme shyness and inability to talk to attractive women. It works (with side effects), but when it comes to asking Missy out the drug wears off and he is unable to speak to her, forcing him to leave defeated. Adding insult to injury, it seemed Missy would have given him a chance, saying when she answered the door that she was 'hoping he'd show up'.

When the boys dine at Penny's restaurant, she learns Leonard has never had a birthday party, as his parents didn't believe in celebrating anything but achievements. She decides to throw him a surprise party, and blackmails Sheldon to join her. However when the day of the party arrives, she finds Sheldon did not buy a birthday gift. After some help from Wolowitz in convincing him it was a 'non-optional social convention', Penny drives Sheldon to a computer store where he becomes distracted, helping other customers with their tech-related questions.

Meanwhile, Wolowitz is in charge of keeping Leonard busy until the party. After Wolowitz pretends to eat a peanut bar (to which he is allergic), Leonard drives him to the emergency room. Due to Sheldon's absent-mindedness, Wolowitz has to keep Leonard distracted even longer, and after a promise from Penny of "being shown which of her friends is easy", Wolowitz actually eats the peanut bar...causing him to swell up in an allergic reaction.

After being at the hospital for several hours (in which they tested everything for fear of being sued), Wolowitz and Leonard arrive back at the apartment to find Penny asleep on the couch, Sheldon asleep in the chair, and Koothrappali drunk singing karaoke telling them everyone left an hour ago. In the end, Penny shows Leonard a cell phone video of the party and ends up giving him a kiss.

Penny breaks up with her boyfriend after he posted intimate details about her on his blog. Leonard goes to comfort her and in doing so accidentally convinces her to get back with her boyfriend. As Leonard laments the situation to the guys, Penny returns and screams at Leonard, saying he made things worse. He tries again and she apologizes; saying it wasn't his fault. She explains that when she went over to make up, she found her boyfriend had already moved on.

As Penny laments her choices in men, Leonard awkwardly asks her on a real date. Penny thinks for a moment and agrees saying, "Sure, why not?". A few days pass and both Leonard and Penny are worried that this date will ruin their friendship. They both seek Sheldon's advice. Sheldon advises Penny that “just like Schrödinger's cat being alive or dead at same time” her date with Leonard currently has both “good and bad” possible outcomes. The only way to find out is to “open the box”, or in other words to go on the date and find which outcome it is. Leonard is also given the same advice, and understands it right away.

When Leonard arrives to pick Penny up for their date, she says they need to talk. Leonard asks her if she has ever heard of Schrödinger's cat and when she says she has, he passionately kisses her. Penny says "the cat's alive" and they leave for dinner.

Meanwhile, Sheldon asks Wolowitz to teach him Mandarin as he's convinced the Chinese restaurant is selling 'Orange Chicken' as 'Tangerine Chicken'...a lesser citrus. He doesn't become very good at it, and as a result the owner (James Hong) thinks he's crazy due to his humorous mistranslations (such as "Show me your mucus!", "This is not a tangerine bicycle!", and "Oxen are in my bed! Many, many oxen!"). It appears Leonard and Penny were going to go to this restaurant on their date, but when they see Sheldon fighting with the owner, they leave before entering.

Penny and Leonard return from the restaurant while Howard and Rajesh spy on them, using a webcam on the corridor connected to a laptop. They try to convince Leonard that the date wasn't working well: they think that when Penny said "let us slow down a bit", it meant that the "fish is bad". But they are obviously envious.

On the other hand, after Penny and Leonard's date, Penny feels uncomfortable having not completed community college. She is afraid that Leonard would become bored with her. She lied to Leonard about completing her own college and makes Sheldon keep her secret. This request to deceive Leonard is very hard for Sheldon (and his facial expressions) and results in the drastic decision on Sheldon's part to move out. However, after moving from Raj's and Howard's homes (in the course of one night), Sheldon is brought back home by Howard. Having unknowingly taken a dose of Valium, Sheldon tells Leonard about Penny's failure to complete community college. Leonard, in an attempt to help Penny, offers her some pamphlets regarding some local community colleges. She promptly slams the door in his face in annoyance. The episode closes as Leonard remarks "Okay this time I know where I went wrong." and he finds Wolowitz and Koothrapalli eavesdropped on the whole incident with the webcam.

The guys return from a Renaissance fair and see Penny with a new guy, Eric (Travis Schuldt), prompting a jealous Leonard to pursue a relationship with fellow physicist Leslie Winkle. All the guys as well as Leslie humiliate Leonard for his apparent failure to keep Penny.

Leslie eventually agrees to date Leonard, repeatedly assuming the dominant role before conceding that he should "assume the male role". Leonard convinces Sheldon to leave the apartment, to give his roommate and Leslie some privacy. On the staircase, while playing Super Mario 64, Sheldon explains to Penny that he feels uncomfortable with Leonard dating his arch enemy, Leslie, who is a "sloppy, arrogant, sub-par scientist".

Sheldon returns to their apartment and formally approves the relationship between Leonard and Leslie, graciously overlooking that she actually believes loop quantum gravity instead of string theory. Leslie gets emotional and makes statements about Lorentz invariance and black hole entropy which Sheldon scoffs at. Leslie expects Leonard to back up her beliefs but finds that he shares Sheldon's instead. Leslie then asks how they should raise the children as if it were a decision to raise their children in one religion or another. After learning about this "deal breaker" which she believes is more serious than Leonard's food allergy and shortness, she breaks up with Leonard. After which Sheldon tells Leonard to look on the bright side Leonard then replies with "What's the bright side?" Sheldon states that it is only 9 more months to Comic-Con and the episode ends. (Near the start it is revealed that at Comic-Con, Raj made out with an attractive man dressed as a Orion slave girl.)

Sheldon becomes distracted from playing Age of Conan by Penny who has accidentally attempted to use her car keys to open her apartment door. Sheldon learns that Penny is frustrated by her failures in life specifically her not getting into any of the parts in plays she's tried out for, not having sex for six months, not receiving a raise at work and, on her way up the stairs, she swallowed a fly. Sheldon invites her into his apartment to wait until the locksmith arrives to open her door. Penny becomes curious about the game Sheldon is playing and quickly becomes addicted. They quickly try to help her regain her regular life back as they try to set her up with an online dating service which failed miserably. After falling deep into her addiction, she inadvertently accepts an online date consisting of questing and drinking ale at the local virtual tavern with Howard. She quickly realises what she's done and promptly stops playing the game as the episode ends.

Raj is included in People magazine's '30 Under 30 to Watch' list, sparking jealousy from his friends, first from Wolowitz who receives no mention of his mechanical design that led to Raj's discovery, and Sheldon who doesn't understand why Raj's work is being recognized (at all). Leonard admonishes them and they attempt to be supportive of Raj, however his antics - including hiring an assistant and misleading the interviewers about his 'humble background' (his father is a gynecologist and drives a Bentley) - leads to a strain between them. Leonard, still having been supportive, loses his cool when Raj asks Penny to be his date for the party thrown by People and makes the belittling comment 'I can't believe it took you a year'. After the party, a very intoxicated Raj and annoyed Penny return to his apartment (The 'Raj Mahal') and receive a web-call from his parents, who criticize Raj for 'dating' a white girl despite Penny setting them straight that she isn't his girlfriend. The night ends when Raj exits to be sick from drinking too much at the party.

The following morning, Raj appears sober and hungover on Penny's doorstep and she makes him apologize to her without any notes or third-party interpretation. This leads to Raj's first ever direct words to Penny while not drunk or distracted - 'Sorry'. She gives him a hug, showing her forgiveness, at which point Leonard leaves his apartment and sees them hugging. When Penny leaves, Raj turns around and gives Leonard a gloating grin. Sheldon comes out and closes the episode by giving an overforced smile he used throughout the episode.

This episode also includes a cameo from Charlie Sheen; at Penny's restaurant after the guys all leave, Raj says to the patron at the next table over, "Hey, buddy. I'm going to be in People magazine!", Mr. Sheen turns around and says "Yeah? Call me when you're on the cover."

Leonard is working nights on an experiment - the lasers he needs are only available on the graveyard timeslot - resulting in his inability to drive Sheldon to the university. As a result, Sheldon must find another way to get to school, which includes invoking a favor from Penny on her day off from the restaurant and a return trip from Wolowitz on his scooter and Raj being forced to pick Sheldon up after both eject him from their respective vehicles. The friends finally stage an intervention and tell Sheldon he must get his driver's license because he should be taking on the responsibility of driving himself to work (and so he will stop bothering them for rides).

After an eventful trip to the Department of Motor Vehicles, resulting in Sheldon circumventing the test procedure by annoying the DMV worker, the gang sets up a simulation for him in the living room. The ensuing practice shows the gang that both the simulation was a great idea as opposed to an actual trip in a car and why Sheldon never learned to drive in the first place - he's a horrible driver. Later, despite actually having a license, Sheldon decides to forgo using it and instead moves into his office for the duration of Leonard's night shifts. However, Leonard slyly neglects to inform him of the actual end date, resulting in Leonard living in peace in the apartment for an extra week as the episode folds.

After an open house night with grad students, a particularly obsessive grad student named Ramona Nowitzki (Riki Lindhome) takes a shine to Sheldon and insinuates herself into his life and work as a girlfriend/assistant. However, Sheldon remains oblivious to her intentions, leaving the gang doubly confounded. She proves to be a positive and helpful influence on his life...until she begins to cut off his social life - no more Halo night, paintball, comic books or Battlestar Galactica. Ramona also goes so far as to presume that Penny has a crush on Sheldon when she sees them talking together in the hallway adjoining their apartments. Sheldon finally realizes that he's in a "relationship" with Ramona and seeks help from Leonard to get rid of her. Things come to a head when the gang comes into the apartment and see that Sheldon's solved his scientific problem that Ramona made him work on day and night until he solved, and when Ramona suggests they share credit for his discovery - calling it the 'Cooper-Nowitzki Theorem', which is what the title refers to - Sheldon's response is 'Get out!'. Things seems to go back to normal until Sheldon is approached by another young woman in the same way Ramona approached Sheldon first, and the episode folds as the cycle begins again.

When Penny's cable gets cut, she starts coming over to the guys' apartment to watch America's Next Top Model. However, after Sheldon bans her from the apartment for a series of petty infractions (touching his food, sitting in his spot, and sending him "email humor"), Penny taunts him at the Cheesecake Factory. After Sheldon goes to the extreme of hanging her laundry from a telephone wire outside the apartment, Penny gets her revenge after Leonard gives her Sheldon's Kryptonite--his mother's phone number. Sheldon's mother calls him and forces him to apologize to Penny.

Meanwhile, Howard and Raj are trying to find the location of the Next Top Model house. They use all of the technology at their disposal, and then some, and finally pinpoint it. They arrive at the house posing as cable guys and work their way in. The episode ends as we're left to wonder what happens next. Analeigh Tipton and Samantha Potter of Cycle 11 guest star.

A smitten Wolowitz thinks he's found his soul mate--until the woman, Dr. Stephanie Barnett, (Sara Rue) meets Leonard. Wolowitz invites Stephanie to drive the Mars rover, which winds up stuck in a ditch. Eventually, to hide evidence of what he has done, Wolowitz destroys all the security tapes and hard drives he can find relating to the Mars Rover mission. Leonard has a hard time deciding how to break the news to Howard that he is actually the one dating Stephanie. Sheldon, of course, is no help. Leonard decides to go to Howard's house to tell him himself. While there, Stephanie calls Howard and gives him the news. Howard tells Leonard "you are dead to me," which lasts until Stephanie invites Howard on a double-date. At the end of the episode, Wolowitz hears the news on TV that the Mars rover has found life on Mars in the bottom of the ditch, but as so much data has been mysteriously lost no one will ever know who made the amazing discovery. The episode closes with Wolowitz remarking "Son of a bitch!"

Sheldon is concerned that Stephanie is the only girl Leonard has dated that he "finds tolerable". He tries to force himself into conversations with "urban slang" to "help" Leonard in the relationship with Stephanie, as "history suggests" he will fail ("You're [Leonard is] Kirk, I'm [Sheldon is] Spock, Wolowitz is Scottie, Koothrapalli is the guy that always gets killed…and now we've got McCoy [Stephanie]"). Leonard attempts to get away from Sheldon by going to the movies with Stephanie, but Sheldon is able to follow them despite Leonard only leaving a note. ("If you were trying to make it impossible to locate you, you couldn't have done a better job." "Oh, clearly I could have.") After believing that Leonard is losing Stephanie, he runs to Penny for advice. Penny, however, is no help, so Sheldon runs to Wolowitz and Koothrapalli, but all they give is, in Sheldon's words, either "apolcalyptic genocide" or "go easy on the cologne". Sheldon once again runs to Penny, but she refuses to talk. Afterwards, Sheldon attempts to prove Leonard's physical superiority to Stephanie by having him open a jar of white asparagus. However, Leonard shatters the jar by accident and has to get stitches. Wondering why Leonard can't supposedly keep a date, Sheldon remarks, "You're seem like a perfectly pleasant person. I can't understand why women have such a hard time loving you."

Following the trip to the hospital, Leonard finds his Facebook status has been changed to "In a relationship". Leonard also finds the cause: Sheldon hacked his account. Leonard forbids him to speak until Stephanie changes her Facebook status to "In a relationship" to match Leonard. Sheldon closes the episode by stating "If I am permitted to speak again, Dr. Sheldon Cooper for the win."

Stephanie has moved in with Leonard and Sheldon, according to their roommate contract, though Leonard denies that she is living with them. She tries helping Sheldon with a ringing in his ear, but is unable to help him. After Sheldon walks Leonard through the changes that must be made now that Stephanie has officially moved in, Sheldon tries to have Stephanie check him more thoroughly, but Penny shows up and it turns out that neither Leonard nor Sheldon ever told Stephanie about Penny. The morning does not go well following that; Penny then proves to Leonard that Stephanie is living with him. Stephanie also buys Leonard new clothes to wear that Leonard ends up hating and eventually ruining. Realizing that his relationship with Stephanie is moving too fast for him, Leonard runs to Penny for advice. However, whenever Leonard tries to follow Penny's advice to tell Stephanie how he feels, they instead end up having sex. This happens again, then Leonard attemps to convince Stephanie by way of a text message only to end up saying "looks like I was wrong" and rushing out of the room.

Meanwhile, Sheldon sneaks into the hospital to test himself multiple times, as apparently the ringing hasn't stopped. By the time Leonard gets back, Stephanie has diagnosed him with an inflamed larynx. The episode ends as Sheldon "asks" Penny with a computer for herbal tea & honey.

Leonard meets his intellectual superior in award winning scientist David Underhill (Michael Trucco). Underhill is the total opposite of Leonard: handsome, charming, and cool. He's the whole package of brains and looks which initially doesn't sit well with Leonard. Although when Hill asks for Leonard's help in research he is undertaking, he happily jumps at the offer. Leonard's joy of working with Hill is hampered by jealousy when Underhill takes an interest in Penny and the feelings are reciprocated.

Sheldon, having always viewed the holidays as a pagan ritual that is rooted in obligations rather than holiday cheer, is taken for a loop when Penny brings him a Christmas gift. He now feels obligated to buy her a gift and enlists Wolowitz and Koothrappali to help him with this task which they reluctantly agree to. Sheldon believes he has come up with the most logical gift giving scenario until he opens Penny's gift; a napkin autographed by Leonard Nimoy.

When Penny's comment about Wolowitz's love life sends him into a depressed stupor, his friends lose their most important team-mate in an upcoming fighting robot competition. When Sheldon accepts a challenge for a one on one fight, pitting their robot, M.O.N.T.E. (Mobile Omnidirectional Neutralization and Termination Eradicator), against that of their co-worker, Barry Kripke (John Ross Bowie) Leonard tries to get Howard back on their team by sending Penny to apologize to him, reminding her that she owed him a favor for when he came home pantless in the Pilot episode. Penny goes to Howard's house to apologize, and ends up hearing his entire dating history. She tells Howard that if he didn't try so hard to get girls, then he would probably do much better in his relationships. After Penny's comforting words, Howard tries to kiss her, immediately followed by Penny punching him in the face. Meanwhile, M.O.N.T.E. loses to Kripke's superior robot, is destroyed beyond repair, and never enters the competition. After Sheldon organizes a vigil for M.O.N.T.E., Penny says it's "just a toy robot" and the episode folds as Sheldon sulks into his room.

Socially awkward, Sheldon develops a scientific procedure for making friends. He needs to use an open science grid computer that is in Barry Kripke's office, but Leonard tells Sheldon that only Barry's friends can use it. After repeated rebuffs from Kripke, Sheldon finds a children's book in the bookstore (where he is hurried out by Leonard who finds him innocently chatting to a little girl in view of a security camera) about how to make friends. He designs a flowchart for accomplishing this task. He goes rock climbing with Kripke but faints halfway up when he sees how far he has come. Sheldon invites him over to his apartment, much to the disgust of everyone else. Raj is informed he is being replaced by Kripke as five friends would be too many to handle. Barry then tells Sheldon there is a sign-up sheet for the computer and Barry has no control over it. Sheldon then dismisses Barry as his friend and goes back to his other four friends: Leonard, Howard, Raj, and Penny.

Sheldon's "simple" solution to Penny's financial problem leads to a confrontation between Leonard and Penny's hulking ex-boyfriend, Kurt. Penny hasn't paid her rent, so Sheldon loans her some money. But then Penny starts to worry about not being about to pay Sheldon back soon enough, even though Sheldon never doubts Penny will pay him back and is not worried about it. Leonard tries to help Penny solve her problem and in the process learns that Kurt owes Penny $1800. Without telling Penny, Leonard goes to Kurt's place to collect the money, and Kurt writes "I owe Penny $1800" on Leonard's forehead. At the end of the episode, Penny pays Sheldon back with money that Kurt paid her, but much to Leonard's disappointment, Kurt evidently never told Penny about Leonard's visit.

A phone call from Leonard's mother, Dr. Beverly Hofstadter (Christine Baranski), leads to problems all around as childhood problems emerge from both Penny and Leonard. Penny bumps into Leonard's mom while doing laundry, and she reveals she is a psychiatrist and neuroscientist. Penny asks about Leonard's childhood, and his mother replies that he was 'in the phallic stage of psychosexual development, grabbing his penis and not letting go'. Penny tells Leonard's mother that she is an actress. By the time they reach Leonard's apartment, Penny is in tears after being psychoanalysed. Sheldon, on the other hand, instantly develops a comfortable relationship with the elder Dr. Hofstadter, resulting in the two discussing 'intimate' relationship points only to lead to a game of Rock Band. In the cafeteria at work, Howard thoroughly enjoys asking Leonard's mom about her other two, more successful children. Leonard gets his revenge by telling her that Howard lives with his mom and about Raj's selective mutism. She decides they are in an ersatz homosexual relationship. Later, Leonard and Penny seek comfort and discuss Leonard's mother first over white wine, then tequila. As the night progresses, Leonard tells Penny that he built a hugging machine when he was ten so he could get physical affection (his dad used to borrow it). Penny increases the stakes, resulting in a kiss. They eventually go to bed together, where Leonard tells her what his mother's reasoning for their impending sex would be: psychologically, he is having sex with his mom and she with her dad. Penny immediately throws Leonard out.

A paintball game leads to a fight between Penny and Sheldon, and romance for Wolowitz and Leslie (Sara Gilbert). Penny accidently shoots the paintball gun at Sheldon's couch cushion (his "spot", his "only point of consistency in an ever-changing world"), and she and Leonard have a hard time getting it cleaned. They try just turning it over but Sheldon still notices the difference and turns it over to find the green stain. Penny has the cushion dry-cleaned, but it takes a week and Sheldon goes through withdrawal during that time. After refusing to sit on the dry-cleaned cushion, Leonard reveals that the Szechuan Palace (the only place Sheldon has his cashew chicken from) closed 2 years ago. Sheldon collapses into the couch but still doesn't like it. Sheldon gets his revenge by shooting Penny at the beginning of the group's next paintball game. Penny then shoots Sheldon, he protests she's dead. Leonard agrees and shoots Sheldon himself, who promptly fires back. The others leave. Meanwhile, Leslie and Howard start sleeping together at Howard's home, which lead to funny exchanges with Mrs. Wolowitz. Leslie is able to give Howard's work project some much-needed funds, although it turns out to be her way of controlling him. She asks him to come to her sister's wedding, he says no and she threatens to take away a trip to the CERN Large Hadron Collider project in Geneva. After getting over his initial reluctance to be in that sort of relationship, Howard accepts his "sex toy/arm candy" role.

A train trip to San Francisco takes a major detour when Leonard, Sheldon, Wolowitz, and Raj discover that their favorite sci-fi actress in all the land (Summer Glau) is on board. But the fanboy frenzy quickly gives way to a heated mass debate when they realize one of them will have to approach her. They took the train because the vote was 3 to 1 for flying, with Sheldon the one vote for the train. While Wolowitz tries to think of a clever opening line for Summer, Raj starts talking to her after drinking what he later discovered was non-alcoholic beer. When he discovers this, of course, he quits talking. Wolowitz then takes over and creeps Summer out. Leonard eventually gets his chance, but Summer gets off the train before he can introduce himself.

In a subplot, Sheldon needs Penny to find his flash drive so she can email him a paper he wants to show Professor George Smoot (as himself) in San Francisco. Leonard asks Sheldon why he can't just e-mail the paper when he returns, and Sheldon replies that he "wants to see [Smoot's] face light up when he reads it." Sheldon initally responds with hostility to Leonard's suggestion that Penny should retrieve the flash drive, reminding Leonard that "no one goes into my room". Sheldon relents, however, and spends most of the episode giving Penny meticulous instructions as to how to enter his room (of course, stipulating that he is granting her access for this one time only). The instructions lead to a Japanese puzzle box containing the flash drive. Sheldon attempts to give Penny instructions as how to open the box, but she instead throws it on the floor and smashes it open. After all of this effort, Smoot wasn't impressed with Sheldon's paper, asking him "Dr. Cooper, are you on crack?"

The title refers to the guest appearance of two stars: Summer Glau of Terminator: The Sarah Connor Chronicles and George Smoot who is famous for the discovery of observational traces of decoupling in physical cosmology.

Penny's home hair-accessories business becomes a complete nightmare when a caffeine-addled Sheldon takes over. Sheldon offers his advice of how Penny can make more money, and then Leonard, Howard, and Raj add their own ideas to mass-produce and sell the products, which are hair barrettes called Pennyblossoms. Leonard makes a website to market and sell the products, and puts in a one-day rush shipping option because "Amazon has one-day rush." Right away, they get an order for 1000 Pennyblossoms from the "East Rutherford, New Jersey Gay/Lesbian/Bisexual/Transgender Alliance." All 5 work all night to finish, only to see the same group had ordered 1000 more, leading everyone to quit because they are tired. All except Sheldon, that is, who is now wide awake and hyper because he was given coffee by Penny. At the end of the episode, Sheldon in his The Flash costume runs out to get more coffee.

Penny develops a rivalry with a sexy female neighbour, Alicia, (Valerie Azlynn) who threatens to become "The New Penny" in the building. The new woman has Leonard, Wolowitz, and Raj all "helping" her set up the apartment, from fixing her CD player to painting the walls. She also offers to pay them in Chinese food, which Penny already bought for the guys. Penny confronts her in the laundry room after she asks Penny how much money physicists make, and they get into a catfight in the lobby, resulting in a black eye for Penny. Meanwhile, Sheldon tries his hand at sarcasm with Penny when she enters his apartment without asking his permission.

While Stuart (Kevin Sussman), a fellow comic book-loving pal of the guys romances Penny, Leonard and Wolowitz venture out into the world of "Ladies' Night" at a local bar.