List of The Big Bang Theory episodes (season 1)

After an unsuccessful visit to the high-IQ sperm bank, Leonard (Johnny Galecki) and Sheldon (Jim Parsons) return home to find out Penny (Kaley Cuoco), a new neighbor, has moved in across the hall. Leonard immediately becomes interested in her, while Sheldon feels his friend is chasing a dream he'll never catch. Later, Leonard invites Penny to his and Sheldon's apartment, where she gets to meet their friends Howard Wolowitz (Simon Helberg), a wannabe ladies man, and Rajesh Koothrappali (Kunal Nayyar), who is so shy he cannot speak to women, unless under the influence of Alcohol.

Leonard is so infatuated with Penny that, after letting her use their shower, he agrees to retrieve her TV from her ex-boyfriend Kurt (Brian Wade). However, Kurt's physical superiority overpowers Leonard and Sheldon's combined IQ of 360, and they return empty handed and depantsed. Feeling bad for them, Penny offers to take the guys to dinner, in what is the start of a new friendship.

When Sheldon and Leonard go over to Penny's apartment to drop off a box of flat pack furniture that came for her, Sheldon is deeply disturbed when he sees how messy and disorganized her apartment appears. Later that night, while Penny sleeps, Sheldon sneaks into her apartment to clean and organize it; Leonard catches him and reluctantly helps him clean.

The next morning, Penny realizes the changes made in her apartment and becomes furious. Sheldon tries apologizing to Penny but fails after remarking that Leonard is a "thorough and gentle lover". Later, Penny encounters Raj in the hallway and talks to him, finally realizing 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 is devastated thinking she has "rejected" him. The guys convince him to ask someone out at work, so he asks out fellow scientist Leslie Winkle (Sara Gilbert), who preemptively turns him down. As Leonard slips deeper into his depression, which involves 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 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 saw was not her boyfriend, but that she has a habit of having weekend-long casual sex encounters to get over relationships. Excited, Leonard begins to become more confident but, after accidentally hurting himself, Penny takes 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.

Recurring characters: Sara Gilbert as Leslie Winkle

Sheldon is fired from his job as a physicist at the university when he insults his new boss Dr. Eric Gablehauser (Mark Harelik). His change of circumstance sends him into a downward spiral of depression which Leonard tries to cure by sending for Sheldon's mom, Mary Cooper (Laurie Metcalf). When she arrives for a visit, the men realize she is completely the opposite of what they expected: she is sweet and down-to-earth, if culturally ignorant, and a devout Christian. After waiting it out, Mary finally forces Sheldon to apologize, and he is given his job back after she flirts with Dr. Gablehauser. As Mary is a widow, Sheldon asks if Dr. Gablehauser will be his new daddy and his mother replies, "We'll see..."

Recurring characters: Laurie Metcalf as Mrs. Cooper

While eating at the Cheesecake Factory where Penny works, the guys run into Leslie Winkle (Sara Gilbert) and she suggests that Leonard play in her string quartet as they are in need of a new cellist. Later, after the practice in Leonard and Sheldon's apartment, Leslie offers herself to Leonard and they have sex.

Meanwhile, Sheldon is uneasy as to how to act after realizing Leslie slept with Leonard. Later on, Penny congratulates Leonard, and he tries to figure out what Penny's words meant. Leonard eventually decides to pursue a real relationship with Leslie over an imaginary one with Penny. However, when Leonard arrives in the lab, Leslie makes it clear she is not interested in a relationship, and only used him to satisfy her sex drive. Leonard later tells Penny, who walks away with a smile on her face, seemingly happy to hear this news.

Recurring characters: Sara Gilbert as Leslie Winkle

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. At the party, the guys have problems socializing; for example, Sheldon is unable to explain his costume as the Doppler effect, and Raj cannot speak to women. Later, Penny's ex-boyfriend Kurt (Brian Wade) shows up and confronts Leonard, causing him to leave. Penny goes to check on him and apologizes; both drunk and upset, she kisses Leonard before she goes back to the party. Meanwhile, Howard cannot find Raj, who ends up going home and has sex with a woman he had met at the party, who found him to be a "good listener".

Recurring characters: Brian Wade as Kurt

Penny's promiscuous friend Christy (Brooke D'Orsay) from Nebraska is in town, and ends up being seduced by Howard. The pair takes over Penny's apartment, causing her to sleep on Leonard and Sheldon's couch for the night. With Howard gone, Penny takes his place playing Halo 3, causing Sheldon much distress as she is naturally good at it. Furthermore, Howard invites Christy to move in with him (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 is even more distraught when he invites Penny again to play Halo but she declines the offer 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. Note. This is the only episode in the first season where Raj talks with Penny in the room without alcohol.

Note:This episode was the most watched of season 1, with 9.68 millions viewers.

Raj introduces his parents, Dr. and Mrs. V.M. Koothrappali (Brian George and Alice Amter), via webcam to his friends. However, Raj immediately becomes overwhelmed after his parents try to arrange a date with Lalita Gupta (Sarayu Rao), a childhood friend, as he cannot talk to women. Meanwhile, Penny takes up bar tending for some extra money. As she practices mixing drinks with the guys, they discover that after a few drinks 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.

However, on his date Raj becomes very obnoxious, and Lalita ends up leaving with Sheldon, who compliments her by comparing her to Princess Panchali, an Indian princess in a fictional fairy tale. Back from the restaurant, Raj 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.

Recurring characters: Brian George and Alice Amter as Dr. and Mrs. V.M. Koothrappali

Note: This was the last episode aired, prior to the 2007-2008 Writers' Strike.

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 à la the movie Scanners, but has a physical altercation with Leonard. In the end, the fight is captured by Howard and put on YouTube.

Note: This was the first episode aired since November 2007, due to the 2007-2008 Writers' Strike.

Penny was given a part in Rent, though after hearing her sing extremely off-key Leonard lies to avoid seeing her performance so he won't hurt 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 becomes so obsessed with the credibility of his lie, that he has Toby Loobenfeld (DJ Qualls), 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.

Note: This is the first episode where Sheldon's knocking habits are shown.

Penny returns from visiting family in Nebraska and as she picks up her mail from Leonard, she mentions most of her relatives got sick. Sheldon freaks out as, according to Leonard, he is a germophobe. Sure enough, Sheldon becomes sick and instantly becomes the biggest pain possible. As it's evident this has happened before, 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, as he is almost blind without them; Penny catches him and sticks him with Sheldon. As he tries to escape after her, he runs headlong into a wall and nearly knocks himself out. In the end, both Leonard and Sheldon are sitting sick and injured on the couch.

Sheldon is envious when he meets 15-year-old child prodigy Dennis Kim (Austin Lee). Losing faith in his research, Sheldon annoys his friends by criticizing 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.

The others, tired of Sheldon's behavior, decide to distract Dennis with girls of his age. At his welcome party, their plan works and Dennis leaves with a blonde 15 year old. Sheldon resumes his research while the others are perplexed at how easily Dennis was able to get a girl.

The guys decide to compete in a physics bowl after finding out that strong competitors have dropped out. However, they find Sheldon's showboating too much and kick him off the team. Upset, Sheldon vows to form his own team and compete against them. The guys, in need of a fourth member, ask Leslie Winkle to join them; she rejects the offer at first, but reconsiders after finding out they are going against Sheldon, who has made fun of her research many times.

At the bowl, the guys team is called PMS (Perpetual Motion Squad, a pun on Premenstrual syndrome); Sheldon's team is named AA (Army ants, a pun on Alcoholics Anonymous) and is composed of himself, the 3rd floor janitor, the lunch lady, and her son (or butcher). PMS wins the competition when Sheldon, himself unable to answer the last question, won't accept the janitor's answer, which turns out to be correct. The latter explains that despite working in America as a janitor, he was a physicist in the former Soviet Union.

Trivia: When attempting to find a replacement for Sheldon on their Physics Bowl team, Raj suggests 'TV's 'Blossom' (Mayim Bialik), citing her PhD. Bialik would later join the cast of the series as Amy Farrah Fowler in the show's fourth season. Raj also suggest "That girl from The Wonder Years" (Danica McKellar), who would appear in the Season 3 episode The Psychic Vortex.

Recurring characters: Sara Gilbert as Leslie Winkle

Online, Leonard buys a time machine from the 1960 film, The Time Machine, believing it to be a miniature, though it turns out to be the full-sized original. As the men move it into the apartment, they accidentally make Penny miss her shift. Penny furiously lashes out at Leonard for his love of comic books and collectibles, and his nerdy ways. Upset, Leonard decides to get rid of all his collectibles, while his friends try to convince him not to, leading to a confrontation at the stairs. After Penny comes out to see what the noise is about, Sheldon angrily accuses her of hypocrisy, reminding her of her own love of childish things like Hello Kitty, Care Bears, Beanie Babies and My Little Pony. Penny, realising the impact of her words, apologises to him. However, she has a new boyfriend named Mike, and once Leonard finds this out, he reverts to his old ways. In the end Sheldon decides to get rid of the time machine after he has a nightmare that he travels to the future and is eaten by the Morlocks.

The guys see an attractive woman in Sheldon's office, and they become even more surprised when Sheldon introduces her as his fraternal twin sister, Missy (Courtney Henggeler). After meeting 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. Naturally, the men instantly try to hit on her, causing Penny to come to her rescue.

Later, Leonard convinces Sheldon to be protective of Missy, so he can have a chance at her over Howard and Raj; however, much to Leonard's chagrin, Sheldon deems all three unsuitable to be with his sister. 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. Leonard wins first chance to ask Missy out, but she turns him down; Howard tries next, but she turns him down as well; finally, Raj goes to talk to Missy, and it appears that she's been waiting eagerly for him, but at that precise moment the drug wears off and he is unable to speak to her, forcing him to leave defeated.

Title reference: Howard saying he'd kill his rabbi with a pork chop to be with Missy, who wasn't Jewish (hence a shiksa).

When the men dine at Penny's restaurant, she finds out that 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, Penny finds Sheldon did not buy a birthday gift, so she has to drive him to a computer store to pick up a present.

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 he is 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.

Penny is furious and breaks up with her boyfriend after he posted details about their sex life on his blog. Meanwhile, Sheldon asks Howard to teach him Mandarin as he's convinced the Chinese restaurant is selling "Tangerine chicken" yet is really using oranges, a cheaper citrus.

As Penny laments her choices in men, Leonard awkwardly asks her out. A few days pass and both Leonard and Penny are worried that this relationship could ruin their friendship. They both seek Sheldon's advice, who uses the "Schrödinger's cat" experiment to explain that at this time, 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 passionately kisses her and she concludes that "the cat is alive" and they leave for dinner.

In the end, Leonard and Penny are seen going to the Chinese restaurant, but they leave before entering as they realize Sheldon, in mangled Mandarin, is fighting with the owner (James Hong).