The Big Bang Theory (season 4)

Howard uses a robotic hand for masturbation, but has to go to a hospital when it malfunctions. Penny learns about Amy, and Sheldon's plan to conceive a child with her by in vitro fertilisation. Penny then urges him to become intimate with Amy first. She accompanies them on their first date, but the conversation quickly turns to a statistical analysis of Penny's sexual history. Afterwards, Sheldon is still intent on having a child, but cancels the plan when Penny threatens to tell his mother.

Sheldon worries that he will not live long enough to experience the technological singularity, so he tries to adopt a healthier lifestyle through diet and exercise. When that proves to be difficult, he hides in safety of his bedroom, interacting with the world through a remotely-controlled computer monitor on wheels (which Penny labels Shelbot). When Shelbot accompanies the men to the Cheesecake Factory, they see Steve Wozniak at another table, who offers to sign Sheldon's vintage Apple II computer. Sheldon emerges from his bedroom to rush the computer to the restaurant, but breaks the computer and an ankle falling down the stairs.

Sheldon and Amy are very compatible with each other, but the rest of the group is irritated by their combined personalities. They are relieved when Sheldon and Amy break up after an argument over the relative significance of physics compared to neuroscience, but Leonard becomes worried when Sheldon brings 25 cats into the apartment. He calls in Sheldon's mother, who manipulates Sheldon into reconciling with Amy.

Howard resumes his relationship with Bernadette after Penny points out that she is his best chance of being with a real woman, as opposed to masturbation. Meanwhile, Sheldon and Raj get into a dispute when Raj brings a large desk into their small office.

When Amy decides to introduce Sheldon to her mother, he becomes worried as being introduced to Amy's mother would indicate that Amy wants to be his girlfriend, which he is not ready for. So he begins to avoid Amy, removing all possible contact with her and even disguising himself so that she would not recognize him. However Amy catches Sheldon outside the apartment building and then calms him down, assuring him that she only wants to introduce him to her mother so that she would be convinced that Amy is in a relationship and leave her alone. Later when talking to Amy's mother, a now relieved Sheldon and Amy lie to Amy's mother that they are in a "sexual relationship", which does not go too well with her. Meanwhile, as Howard now has a girlfriend, Leonard decides to invoke the "Girlfriend Pact" (from the Season 3 episode The Creepy Candy Coating Corollary) with Howard. Howard and Bernadette set up a double date with Leonard and one of Bernadette's friends, Joy. Joy behaves in an obnoxious and inappropriate manner during the date, which disgusts Leonard. However he later accepts another date with Joy due to understanding that sex is a strong possibility.

Raj's younger sister Priya visits Pasadena. Though unknown to the others including Raj, Priya had been involved in a sexual encounter with Leonard five years earlier, despite Leonard's assurance to Howard that he would never hit on Priya in deference to Raj. That night, Priya sneaks into Leonard and Sheldon's apartment unknown to Sheldon and has sex with Leonard. Sheldon finds out the following morning and frames an elaborate, flawless lie to cover it up. But Leonard decides not to hide the truth and confesses to Raj that he slept with Priya. Raj is initially upset, but reconciles with Leonard after he says that Priya broke his heart. The guys then unleash confessions of all of the deceptions they have perpetrated on each other.

Howard needs security clearance to work on a new project, so an FBI agent comes to ask Leonard, Sheldon and Raj about his background. Since the agent is an attractive woman, Raj and Leonard embarrass themselves. Sheldon, however, tells her about the time Howard drove a Mars rover into a ditch (The Lizard–Spock Expansion), so Howard is rejected for the project. Howard rejects Sheldon's apologies until he is allowed to sit in Sheldon's spot on the couch, although Sheldon takes it back after 94 seconds.

The men try to attend a screening of "Raiders of the Lost Ark" with 21 seconds of previously unseen footage, but are displaced by Wil Wheaton and his friends cutting in front of them. Sheldon then steals the film from the theater. Meanwhile, Amy gets herself invited to a "girls' night" with Penny and Bernadette. She turns out to be an awkward participant, while Penny becomes defensive when the others ask her why she ended her romance with Leonard.

Penny's father, Wyatt, has come to visit. Since he feels that Leonard is the best man that Penny ever dated, she asks Leonard to pretend that they are still together. Leonard happily plays along until Penny confesses to Wyatt. Wyatt is angry that Penny does not trust him, and pleads with Leonard not to give up on her. Sheldon stays up all night amending the roommate agreement to address her personal habits, and becomes angry to learn that he needn't have bothered. Meanwhile, Howard and Bernadette stay with Raj in his telescope control room. He becomes drunk and complains that he will never kiss a woman. When Bernadette expresses sympathy, he leans in to kiss her, but ends up kissing Howard.

Amy meets Penny's ex-boyfriend Zack and experiences sexual arousal. She later discusses her symptoms with Sheldon, though Sheldon prefers to assume that Amy has contracted an alien parasite. Later, when Sheldon discusses Amy's situation with Penny, she tells him that there is "something" he could do about Amy's "urges", meaning that he should have coitus with Amy, but Sheldon assumes that she meant Amy and Zack should have coitus. He arranges for them to meet again, but Amy discovers that Zack is not very intelligent, solving the problem. Meanwhile, Howard and Raj argue about, if they both acquired superpowers, which of them would be the sidekick. They try to settle the issue in a wrestling match refereed by Leonard, but they merely circle around threatening each other.

Penny gets back together with Zack. The guys insult Zack's low intelligence which upsets him; however he reconciles with the four and accompanies them to the comic book store. The five enter a costume contest and urge Penny to join in a Wonder Woman costume. She refuses until Leonard talks to her. She reveals that she got back together with Zack only because she did not want to spend New Year's Eve alone. Leonard questions why not with him, prompting Penny to suddenly remember the fun times she used to have with him, convincing her to change her mind. At the comic book store, the group win the award for the Best Group Costume, although it is hinted that they only won because they included a female Wonder Woman. The episode concludes with the six witnessing an ongoing car theft and then scurrying off cowardly despite wearing Justice League costumes.

Leonard gets an idea for a differential equation app, but Sheldon quickly tries to take charge of the project. After he is rejected by the others, he tries to interfere with the project by loudly playing a theremin. When Penny finds him sadly playing his theremin in the building lobby, she tries to get him reconciled with the other men. When that doesn't work, he reluctantly helps Penny create an app for shoe shopping.

Amy asks Penny to join the group at a science conference in Big Sur. Things go fairly well until Howard meets Glenn, a large handsome man that Bernadette used to date. Howard's insecurities anger Bernadette, setting off a chain reaction of bed swapping that causes the next day's science panel to become a series of personal recriminations, much to Sheldon's surprise. He tries to get the panel back on track by asking for questions from the audience, but the only question is from Penny for a ride back to Los Angeles. When Glenn offers to give her a lift, Leonard instantly objects. When the group drives back home (without Penny), everyone except Sheldon is in a foul mood.

When Sheldon is unsuccessful as a lecturer due to his condescending personality, Amy suggests that he take acting lessons from Penny. Instead of using a professional script, he insists that they use a Star Trek fanfiction script which he wrote as a child. Penny plays the role of Spock as she feels that Sheldon needs to come out of his comfort zone, while Sheldon plays the dual role of himself and his mother. However while acting out the script, Sheldon immerses himself so deeply in the part that he is brought to tears, because he does not want to be taken away from his mother by Spock. Penny then calls his mother and gives the phone to Sheldon so that his mother can console him. Meanwhile, Raj becomes infatuated with Bernadette and fantasizes about them getting together.

President Siebert invites the men to a fundraising event at Caltech. While Leonard, Howard and Raj are willing to attend, Sheldon feels that such events are beneath him. He changes his mind when Amy points out that, without his participation, any monies raised might go to programs other than physics. When he finally arrives, Siebert quickly regrets inviting him. Meanwhile, Leonard makes a good impression on the considerably older Mrs. Latham. The next day, she picks him up in her limousine, and makes it clear that she will only donate the money if Leonard has sex with her. He refuses, but everyone else urges him to go through with it. After he does so and she donates the money, Leonard receives a standing ovation at work.

Priya returns and resumes dating Leonard, despite Raj's protests. Amy gets Penny to admit that she is extremely jealous of Priya, giving her the chance to measure the brainwaves associated with jealousy. Meanwhile, Bernadette is tired of Howard sneaking out of bed quickly to assist his mother and insists that he move in with her. He eventually does so, but when she discovers how Howard is completely dependent on his mother and now her to do everything for him, she forces him to move back in with his mother.

Now Leonard and Priya are dating and since she lives with Raj, Leonard and Howard decide to have dinner there instead of the normal venue which disturbs Sheldon. Sheldon stops by the Cheesecake Factory where Penny tells him he has to accept that the gang will be hanging out at Raj's place more often. Amy tells him that Leonard is the nucleus of their social group and not Sheldon. As a result, he sets up a new group consisting of Kripke, Stuart, Zack and LeVar Burton (whom he tweeted) with him as the nucleus. Sheldon's plans are dismissed by the others in favor of Zack's dating stories and getting drunk and singing karaoke. Naturally, Sheldon does not like that very much, so he decides to go back to his old friends, who, in the meantime, realized that they actually missed Sheldon. In the end, LeVar Burton finally does show up at Sheldon's place, but when he sees Kripke, Stuart and Zack singing karaoke together, he leaves immediately. Meanwhile, Bernadette and Amy want to take Penny out dancing to get her mind off of Leonard and Priya. As she changes clothes, she tells the girls that it is the first time in her life she is satisfied with being single and does not want to have sex with a random guy to make up for it, however when Amy finds the encased snowflake that Leonard gave to her after his expedition, she changes her mind as her feelings for Leonard come back to the surface again.

Priya has begun to dominate in her relationship with Leonard, buying him new clothes (which Leonard is not comfortable with). She also feels threatened by Leonard continuing to hang out with Penny, even though they have broken up, so during sex, she tells him to break all contact with Penny. Leonard initially hesitates, but still tries to tell Penny to stay away from him, albeit indirectly. Penny does not get the right message, but Leonard takes comfort in the fact that he tried. Later Penny interrupts Leonard's date with Priya and is completely oblivious to what is going on. Leonard once again indirectly tries to tell Penny to stay away from him, but this time Penny gets the message and realizing that Priya was behind this, decides to break all contact with Leonard. Meanwhile, Howard shows Sheldon, Raj and Penny a card trick in which a volunteer picks a card and after shuffling the deck, Howard tells the volunteer his/her card. Raj and Penny both try out the trick and are amazed by it. Though at first his attitude is dismissive, Sheldon eventually becomes obsessed with discovering the trick's secret, as even Penny could figure it out. He goes to the extent of using a barcode reader in order to recreate the trick. It is then revealed (to the audience) that the card trick is just a prank played by Howard, Raj and Penny on Sheldon. Raj and Penny willingly pretended that Howard was successful, knowing fully well that Sheldon would go nuts trying to figure out the trick's secret. When Sheldon wants Howard to perform the trick on him in the end, Raj manages to get a peek on Sheldon's card and uses hand signals to tell Howard the right answer. Sheldon is puzzled once more, still not knowing that he had been pranked all along, while Howard and Raj both laugh gleefully.

Sheldon's World of Warcraft account has been hacked and all of his WoW stuff have been stolen. He enlists the help of the other guys in finding the hacker. Meanwhile, Amy, Bernadette and Penny criticise Priya and her arrogant, dominating behavior. Penny takes Amy's comments lightly, but after an encounter with Priya in the apartment lobby, she agrees with Amy. Howard manages to trace the hacker and reveals that his name is Todd Zarnecki. The guys then drive to Carlsbad to get back Sheldon's WoW stuff. Todd refuses to return Sheldon's stuff, and snatches the bat'leth that Sheldon had intended to intimidate him with. Defeated, the guys decide to return home, but their car breaks down midway, prompting Leonard to call Penny to fetch them. When Penny hears their story, she not only realises that Leonard did not want to see Priya that night, but also drives them back to Todd's house where she delivers a groin attack and forces him to return Sheldon's stuff.

Bernadette considers breaking up with Howard at the same time that he decides to propose marriage to her. Raj hopes that she will reject his proposal so that Bernadette will become available for dating him. The gossip swirling around these developments intrigue Amy and Sheldon enough to conduct an experiment in memetic epidemiology. They launch pairs of false gossip, only one of which is tantalizing, and measure how quickly they each spread. Bernadette accepts Howard's proposal, disappointing Raj.

When Leonard takes a shower with Priya, he gets charged with two violations of the roommate agreement: Leonard denied access to the bathroom when Sheldon had to urinate and Leonard was not the only person in the shower. However, Priya uses her legal skills to nullify the charges. When the guys take advantage of Priya's skills in order to eat Greek food, which Sheldon does not like at all, on pizza night, Sheldon seeks shelter at Penny's, who invites him to join her on a girls' night out with Amy and Bernadette badmouthing Priya. After the girls have some cocktails and Sheldon mentions he had learned to dance at cotillions as a child, they decide to take Sheldon dancing. After dancing, Sheldon takes a drunken Amy back to her apartment. Amy tells Sheldon to fight dirty against Priya. She then proceeds to kiss him and then rushes to her bathroom to vomit. The next day, Sheldon blackmails Leonard into signing a revised roommate agreement. If he refuses, Sheldon will send an e-mail to Priya's parents about her dating Leonard (a non-Indian). Priya insists that he sign or their relationship is over. He signs.

Bernadette reveals that Howard and her have been invited to a double date by Priya. Amy proposes to use Bernadette to spy on Priya and spread disinformation about Penny and Leonard. Meanwhile, Raj, who is still very lonely, visits Sheldon, who is busy developing a version of three-player chess. He tries to get advice about whether or not he should try to cure his social anxiety using an experimental drug. Sheldon finally advises Raj to take the drug and accompanies him to a coffee shop to observe the drug's effectiveness. It ends up working very well, as Raj starts to talk to a woman named Angela, who also seems to like him. However, Raj loses all his inhibitions and strips completely naked, thus scaring off Angela. In the end, Sheldon, Leonard and Howard play a test game of Sheldon's chess while a still-nude Raj is watching them.

Bernadette and Howard decide to inform their respective parents about their engagement. Howard arranges a lunch for his fiancée and his mother and they end up getting along well. After Howard hears the good news while talking to his mother, who is in the bathroom, he decides to make the move and tell her about the engagement. To his horror, she collapses from an apparent heart attack. Howard takes her to the hospital. When the others hear about this, they rush to the hospital. Sheldon is initially reluctant to visit the hospital and only agrees to come after Penny pressures him to go. When Bernadette finds out that Howard was telling his mother about his engagement before she collapsed, she is very upset, as she believes that she is the reason all this happened. The doctor informs them that it had not been a heart attack and that she wants to see Bernadette. It is revealed that she actually had food poisoning from the restaurant and was worried about Bernadette's well-being, whom she actually likes a lot. Bernadette ends up being mad at Howard, as he led her to believe that it was all her fault. While in the hospital, Priya and Penny start searching for the cafeteria and bond in the process. They mainly discuss Leonard's skills in the bedroom. Sheldon has other things to worry about, as he tries to avoid contact with anything in the hospital. When he tries to avoid a coughing patient, he seeks shelter in a random room. Much to his horror, it turns out to be a bio-hazard room. Sheldon is eventually quarantined in the hospital for two weeks after the exposure, with his friends keeping him company.

Bernadette announces that she is getting her Ph.D. and being offered a well-paid job. Later Bernadette decides to buy Howard an expensive watch as a present and tells him to "let her worry about the money", a comment that disturbs Howard. Meanwhile, Leonard's and Priya's noisy sexual encounters sends Raj to spend the night at Sheldon's apartment. When Leonard finds him in his bed, he proposes that Raj moves in with Sheldon until Priya finds her own place. Raj likes the idea, signs Sheldon's roommate paperwork and moves in with him. He then prepares a fancy dinner for them which delights Sheldon who decides Raj is a much better roommate than Leonard. Penny stops by, stays for dinner and the both of them get drunk from the wine Raj serves. Penny confesses that she should not have broken up with Leonard, and tells Raj she would be "on" him if they were not friends. When Priya receives a video call from her parents, Leonard overhears them saying that she is moving back to India and storms out revealing their relationship to her parents. At the end of the episode, Sheldon discovers Leonard sleeping on their couch, just before Howard shows up after a fight with Bernadette over the watch. Penny wakes up in bed with Raj and becomes horrified when she realizes that she hooked up with him. She tells him not to say anything about the night before and tries to sneak out of the apartment; however, they are busted by the others. Penny just states that it is not what it looks like and leaves, confusing the others.