The Big Bang Theory (season 1)

After an unsuccessful visit to the high-IQ sperm bank, Dr. Leonard Hofstader and Dr. Sheldon Cooper return home to find Penny, a new neighbor who has moved in across the hall from their apartment. Leonard immediately becomes interested in her, but Sheldon feels he is chasing a dream he will never catch. Leonard invites Penny to his and Sheldon's apartment for Indian food where Penny asks to use their shower, since hers is broken. While wrapped in a towel, she gets to meet their friends Howard Wolowitz, a wannabe ladies man who tries to hit on her, and Rajesh Koothrappali, who suffers from selective mutism in the presence of women and is unable to speak to her.

Leonard is so infatuated with Penny that, after letting her use their shower, he agrees to retrieve her TV from her ex-boyfriend Kurt. However, Kurt's physical superiority overpowers Leonard and Sheldon's combined IQ of 360, and they return with a walk of shame, empty handed, and without pants. Feeling bad, Penny offers to take the guys out to dinner, beginning a new friendship between them.

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 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 his invite 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.

Recurring character: 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. 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 (if culturally ignorant) , 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. 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 Mary Cooper and Mark Harelik as Dr. Eric 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 walks away smiling, seemingly happy to hear this news.

Recurring character: Sara Gilbert as Leslie Winkle
Note: This episode was selected as a Primetime Emmy Award submission for Johnny Galecki and Sara Gilbert for Outstanding Lead Actor in a Comedy Series and Outstanding Guest Actress in a Comedy Series, respectively, but they both ended up not receiving a nomination.^[14]

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 his explanation, 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".

Recurring character: Brian Patrick Wade as Kurt

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.

Recurring character: Carol Ann Susi as Mrs. Wolowitz
Guest stars: Brooke D'Orsay as Christy and James Hong as Chen
Note: This is the first episode where Raj talks with Penny in the room without alcohol and Mrs. Wolowitz makes her first vocal appearance. This was the most watched of season 1, with 9.68 millions viewers.

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.

Recurring characters: Brian George as Dr. V.M. Koothrappali and Alice Amter as Mrs. Koothrappali
Guest star: Sarayu Rao as Lalita Gupta
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, 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 taking a picture of her sleeping on his shoulder and posting it to Facebook with the caption "me and my girlfriend".

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

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.

Guest star: DJ Qualls as Toby Loobenfeld
Note: This is the first episode where Sheldon's knocking habits are shown, although with 4 knocks, which changes to 3 knocks in the next season and remains so.

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.

Recurring character: Carol Ann Susi as Mrs. Wolowitz
Note: This is the first episode where Penny sings "Soft Kitty" to a sick Sheldon and was selected as a Primetime Emmy Award submission for Jim Parsons for Outstanding Lead Actor in a Comedy Series, but ended up not receiving a nomination.^[14]

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.

Recurring character: Mark Harelik as Dr. Eric Gablehauser
Guest star: Austin Lee as Dennis Kim

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 (saying of Sheldon, "that arrogant, misogynistic East-Texas doorknob that told me I should abandon my work with high-energy particles for laundry and child-bearing?").

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 (a 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.

Recurring characters: Sara Gilbert as Leslie Winkle and Mark Harelik as Dr. Eric Gablehauser
Note: 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 suggests "That girl from The Wonder Years" (Danica McKellar), coauthor of scientific papers and best-selling math-based books, who would appear in the Season 3 episode "The Psychic Vortex".

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. His hope of a relationship with Penny renewed, Leonard decides to keep his belongings, but when Penny's new boyfriend Mike shows up, he reverts to his old ways. 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.

Guest Star: Andrew Walker as Mike
Note: In this episode, Leonard says that the elevator has been broken for two years, implying that the incident happened in 2006; however, in the season three episode, "The Staircase Implementation," it is revealed that the elevator was destroyed in 2003.

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.

Guest star: Courtney Henggeler as Missy Cooper
Note: This episode was originally aired as "The Shiksa Indeterminacy",^[24] but CBS now refers to this episode as "The Pork Chop Indeterminacy" on its website.^[25]

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.

Recurring character: Vernee Watson as Althea

Penny furiously breaks up with Mike after he posted 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.

Guest star: James Hong as Chen