The Big Bang Theory (season 1)

When Sheldon and Leonard drop off a box of flat pack furniture that came for Penny, Sheldon is deeply disturbed when he sees how messy and disorganized her apartment is. Later that night, while Penny sleeps, Sheldon, extremely obsessive-compulsive and unable to sleep, sneaks into her apartment to clean and organize it; Leonard catches him, but reluctantly helps him clean. The next morning, Penny realizes that they had been in her apartment and becomes furious. Sheldon tries apologizing to Penny but fails after remarking that Leonard is a "gentle and thorough lover". Later, Penny encounters Raj in the hallway. Although Raj cannot talk to Penny, she calms down talking about the problem to him and hugs him, reasoning with herself that the guys were just trying to help her. Leonard then apologizes, prompting Penny to hug him and forgive him.

When Leonard sees Penny kissing a man in front of her apartment door, he becomes devastated, thinking she has "rejected" him. The guys convince him to ask someone out at work, so he asks out fellow scientist Leslie Winkle, who preemptively turns him down. As Leonard slips deeper into his depression, thinking about buying a cat, Sheldon points out that he was not actually rejected by Penny, as he had not asked her out. Leonard agrees and promptly asks Penny out to dinner, but she mistakes it as an invitation to hang with all the guys, so Leonard makes up excuses as to why the guys were absent. On the date, Penny mentions the man Leonard saw was not her boyfriend, but that she has a habit of having weekend-long casual sex encounters to get over relationships. Excited, Leonard regains his confidence, but he accidentally hurts himself and Penny has to take him home. In the end, Leonard does not get the courage to reveal his true feelings to Penny, even though she suspected what he was up to.

Sheldon is fired from his job as a physicist at the university when he insults his new boss Dr. Eric Gablehauser. His change of circumstance sends him into a downward spiral of depression which results in working on developing luminous fish for nightlights and buying a loom for himself. Worried, Leonard calls Sheldon's mother, Mary Cooper. When she arrives for a visit, the men realize she is completely the opposite of what they expected: she is sweet, down-to-earth, a devout Christian, and a really good and loving and caring mother. Mary finally forces Sheldon to apologize, and he is given his job back after she flirts with Dr. Gablehauser.

While eating at the Cheesecake Factory where Penny works, the guys run into Leslie. Leslie wants Leonard to play in her string quartet as a new cellist, as the other players are uncomfortable near the current cellist due to the chance he was irradiated at work. After practising in Leonard and Sheldon's apartment, Leslie and Leonard have sex, making Sheldon uneasy. Penny congratulates Leonard, whom tries to figure out what she meant. Leonard decides to pursue a real relationship with Leslie over an imaginary one with Penny, but when he arrives in the lab, Leslie makes it clear she is not interested in a relationship, and was only using him for a one-night stand to satisfy her sex drive. Leonard later tells Penny, who reassures him and walks away smiling.

The guys are invited to Penny's Halloween party. They originally turn it down as they do not dance but change their minds when costumes are mentioned. At the party, the guys have problems socializing; Sheldon is unable to explain his costume as the Doppler effect since none of Penny's friends are physicists or smart enough to understand it, Raj cannot speak to women, Howard is mistaken for Peter Pan instead of Robin Hood, and Leonard is insecure. Kurt later shows up and confronts Leonard, causing Leonard (and later Penny) to leave. Penny goes to check on Leonard and apologizes while also very upset about Kurt. Leonard tells her that she is perfect and they kiss, though Penny confesses she is drunk. They return to the party and kiss again in front of Kurt, prompting Leonard to say "That's how we roll at the Shire!", before locking the doors in panic. Meanwhile, Howard cannot find Raj, who ends up going home and having sex with a woman he had met at the party, who found him to be a "good listener".

Penny's promiscuous friend Christy from Nebraska arrives in town, and ends up sleeping with Howard almost instantly, who is dragged to her. The pair take over Penny's apartment, causing Penny to sleep on Leonard and Sheldon's couch for the night. With Howard gone, Penny takes his place playing Halo 3 and ends up being naturally good at it, annoying Sheldon. Furthermore, Howard invites Christy to move in with him (and his mother), taking up all his time. Distraught over the odd number in the group, Sheldon invites Penny again to play Halo, but she declines to go dancing. Believing that they are falling apart as a group, the guys go to speak to Howard. When they arrive, they hear Christy and Howard's mother arguing loudly, causing Christy to leave. Howard, once again single, goes with the guys to play Halo and the group is restored yet again.

Raj introduces his parents, Dr. V.M. Koothrappali and Mrs. Koothrappali, via webcam to his friends. However, Raj immediately becomes overwhelmed after his parents try to arrange a date with Lalita Gupta, a childhood friend and dental student, as he cannot talk to women. Howard pretends to be Raj and sets up the date with Lalita by using a fake Indian accent, much to Raj's annoyance. Meanwhile, Penny takes up bar tending for some extra money. As she practices mixing drinks with the guys, they discover that after drinking an alcoholic beverage, Raj can actually talk to women. They decide the date should be at Penny's restaurant, so she and the guys can keep an eye on Raj's behavior while drinking. On his date, Raj becomes a very obnoxious drunk, and Lalita ends up leaving with Sheldon, who impresses her by complimenting her, comparing her to Princess Panchali, a princess in an Indian fairy tale. Raj tries to explain to his parents what happened, who become angry at him for drinking. Leonard asks Sheldon if he is going to see Lalita again and he replies that he already has a dentist.

Cleaning up after an Internet experiment, Leonard discovers a letter in the trash can, which is an invitation he and Sheldon received to present some research they had done together at an academic conference. Sheldon disapproves of presenting research in person, but Leonard defies him, saying he will go 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 tries to discredit Leonard's presentation by attempting to "blow up" Leonard's head with his mind à la the movie Scanners, causing a physical altercation between the two. Howard records the fight on his cell-phone and puts it on YouTube, but earns the ire of Penny after also taking a picture of her sleeping on his shoulder and posting it to Facebook with the caption "me and my girlfriend".

Penny is given a part in Rent, but after Leonard and Sheldon hear her sing extremely off-key (with Sheldon commenting, "if cats could sing, they'd hate it too"), Leonard lies to avoid seeing her performance and hurting her feelings. Sheldon becomes worried that Leonard's lie was not good enough, so he creates an intricate lie about his non-existent drug addicted cousin "Leopold Houston", who was in need of an intervention (and thus why they could not see the play). Sheldon ends up becoming so obsessed with the credibility of his lie that he has Toby Loobenfeld, a research assistant who double majored in Physics and Theatre, impersonate cousin "Leo" to convince Penny that the lie is true. In the end, not only does Leonard still have to see the videotape of Penny's performance, but Penny and "Leo" start flirting as well.

Penny returns from visiting family in Nebraska, but mentions while picking up mail from Leonard that most of her relatives got sick. Sheldon, a germophobe according to Leonard, freaks out and becomes sick, causing his obnoxious personality to increase drastically as he becomes demanding. Familiar with Sheldon being sick, Leonard and the guys hide from him at a Planet of the Apes series marathon, leaving Penny to take care of Sheldon. However, Leonard breaks his glasses and must re-enter the apartment using help with a web-cam from Howard and Raj. Penny catches him and sticks him with Sheldon. As he tries to escape, he runs into a wall and nearly knocks himself out. In the end, Leonard and Sheldon sit injured and sick on the couch respectively.

Sheldon becomes envious when he meets 15-year-old child prodigy Dennis Kim, who was invited to work at the university, and soon realizes that not only is Kim like Sheldon himself (a child prodigy, obnoxious, and arrogant), but is even smarter than he is. Losing faith in his research, Sheldon annoys his friends by criticizing their work more than usual and contributing to their work. Deciding his work in physics is useless, he aims for the Nobel Peace Prize by attempting to solve the Middle East conflicts: he proposes the creation of an exact replica of Jerusalem in the Mexican desert, which ends up in a giant failure. The others, tired of Sheldon's behavior, decide to distract Dennis with girls of his age. At his welcome party, their plan works unexpectedly and Dennis leaves with a blonde 15-year-old, using money to get her with him. Sheldon resumes his research while the others are perplexed at how easily Dennis was able to get a girl. The guys spot Dennis drinking heavily and making out in a park and feel sorry for him, but Sheldon mocks Dennis in victory.

The guys decide to compete in a physics bowl after finding out that the strong competitors have dropped out, but find Sheldon's showboating too much and kick him off the team. Sheldon, upset, vows to form his own team and compete against them. The guys, in need of a fourth member, ask Leslie; she rejects at first, but agrees after finding out they are going against Sheldon, who has often criticized her research and gender. The guys name themselves PMS (Perpetual Motion Squad, an unintentional pun on premenstrual syndrome), while Sheldon's team is named AA (Army ants, an unintentional pun on Alcoholics Anonymous) and is composed of himself, the 3rd floor janitor, the lunch lady, and her son (or possibly her butcher). PMS win when Sheldon, unable to answer the last question, refuses to accept the janitor's answer, which turns out to be correct. The janitor explains that despite working in America as a janitor, he was a physicist in the former Soviet Union.

Leonard buys a time machine online from the 1960 film, The Time Machine, believing it to be a miniature, though it turns out to be the full-sized original. Because the men's efforts to move the cumbersome object into the apartment cause Penny to lose much of her day's time, including a shift at work, she furiously lashes out at them for their childlike preoccupation with various genre memorabilia. Leonard, upset at possibly losing the respect and attention he craves from Penny, decides to get rid of all his collectibles (which Howard refers to as "Nerdvana"), while his friends try to convince him not to, leading to a conflict among the other men over the collectibles of his that they want. Later, Sheldon turns the tables in Leonard's favor, accusing Penny of being a hypocrite by pointing out her own love of things such as Hello Kitty, Care Bears, Beanie Babies and My Little Pony. Penny, realizing his point, apologizes to Leonard, and encourages him not to give up the things he loves before her new boyfriend Mike shows up. His hope of a relationship with Penny renewed, Leonard decides to keep his belongings. Sheldon ends up having a nightmare where he travels to the future and is attacked by Morlocks. He then "wakes up" sat in the time machine and tells Leonard they need to get rid of it. Leonard agrees that the time machine is too big for the living room and has hired some guys to help move it; the Morlocks, who attack Sheldon again.

Sheldon introduces the guys to his twin sister, Missy, who all become attracted to her. Raj decides to take part in a drug trial to cure his inability to talk to women, and Leonard insists she stay with them at the apartment. The men predictably begin awkwardly flirting with her, causing Penny to come to her rescue. Leonard convinces Sheldon to be protective of Missy so that he is more likely to win her heart over Howard and Raj, but Sheldon deems all three unsuitable to be with Missy. Upon learning about Sheldon's plan to find a suitable mate for her, Missy confronts her brother, and Sheldon agrees to let Missy date whoever she wants after she kicks him in the groin. Missy turns Leonard and Howard down. Finally, Raj goes to talk to Missy, and she seems interested in him, but the drug wears off and he is unable to speak to her, forcing Raj to leave defeated.

Penny learns that Leonard has never had a birthday party, as his parents did not believe in celebrating anything but achievements. She decides to throw him a surprise party, and blackmails Sheldon to join her by threatening to draw a smiley face on one of his mint condition comic books. However, when the day of the party arrives, Penny finds Sheldon did not buy a present, so she has to drive him to a computer store to pick one up. Meanwhile, Howard is in charge of keeping Leonard out of his apartment until the party. Howard pretends to eat a peanut bar, to which he is allergic, so Leonard can drive him to the emergency room. However, due to Sheldon's absent-mindedness in the computer store, Howard has to keep Leonard distracted even longer and is eventually forced to actually eat the peanut bar, causing him to swell up in a severe allergic reaction. After being at the hospital for several hours, Howard and Leonard arrive back at the apartment to find the party has ended, though Penny does kiss Leonard.

Penny furiously breaks up with Mike after he posts details about their sex life on his blog, while Sheldon, determined to prove that the Chinese restaurant uses oranges instead of tangerines in their "Tangerine chicken", asks Howard to teach him Mandarin. As Penny laments her choices in men, Leonard awkwardly asks her out. A few days pass and both become worried that this relationship could ruin their friendship, so both seek Sheldon's advice. Sheldon uses the "Schrödinger's cat" thought experiment to explain that the date has both "good" and "bad" possible outcomes, and the only way to find out is to go on the date and find which outcome it is. When Leonard arrives to pick Penny up, he mentions the experiment and passionately kisses her. She concludes that "the cat is alive" and they leave for dinner. Leonard and Penny go to the Chinese restaurant, but they leave before entering as they see Sheldon fighting with the owner in Mandarin, and doing it very badly, disturbing the patrons.