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 out Penny, a new neighbor, has moved in across the hall from their apartment. Leonard immediately becomes interested in her, while Sheldon feels his friend is chasing a dream he will never catch. Later, 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 for them, Penny offers to take the guys to dinner, in what is the start of 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 appears. Later that night, while Penny sleeps, Sheldon, who is extremely obsessive-compulsive and cannot sleep because of the fact, sneaks into her apartment to clean and organize it; Leonard catches him and 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 and talks to him, finally reasoning with herself that the guys were just trying to help her though Raj does not utter a word as he is not able to speak to women, unless drunk, due to his selective mutism. 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, 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 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. Leonard tries to cure him by calling 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 and down-to-earth (if culturally ignorant) and a devout Christian, a really good and loving and caring mother, which they have not expected. 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 Mary Cooper and Mark Harelik as Dr. Eric Gablehauser

While eating at the Cheesecake Factory where Penny works, the guys run into Leslie and she suggests that Leonard play in her string quartet as they are in need of a new cellist as the other players are uncomfortable near the other guy due to the chance he was irradiated at work. 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 was only using him for a one-night stand 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 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, first turning it down as they do not dance but changing their mind 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. Later, Penny's ex-boyfriend Kurt shows up and confronts Leonard, causing Leonard (and later Penny) to leave.

Later that night, Penny goes to check on him and apologizes while also very upset about Kurt. Leonard tells her that she is perfect and she kisses him. Leonard asks her how much she has had to drink, she claims "a lot" and that he was smart for figuring that out. Leonard sarcastically admits "yea, I'm a fricking genius" for stopping Penny's advances. She goes back to the party kissing him 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 has 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 is in town, and ends up sleeping with Howard almost instantly, who is dragged to her. The pair takes 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, 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 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 (using a fake Indian accent, much to the annoyance of Raj). 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.

However, on his date Raj becomes a very obnoxious drunk, and Lalita ends up leaving with Sheldon, who compliments her by comparing her to Princess Panchali, a princess in an Indian fairy tale, and she is very impressed with Sheldon. 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. 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. He and Sheldon had been invited to present some research they had done together at an academic conference. However, Sheldon disapproves of presenting research in person and says Leonard cannot go without him. 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 the things even worse. At the conference Sheldon appears, tries to discredit Leonard's presentation, tries to "blow up" Leonard's head with his mind à la the movie Scanners and then has a physical altercation with Leonard. Howard captures the fight with his cell-phone camera and puts on YouTube, but also earns the ire of Penny because he took a picture of her sleeping on his shoulder and posted 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, though after Leonard and Sheldon hear her sing extremely off-key (with Sheldon commenting, "but if cats could sing, they'd hate it too"), Leonard lies to avoid seeing her performance so he will not 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).

Soon enough, Sheldon becomes 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 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, since he is a germophobe and his obnoxious personality drastically increases when he is sick.

Familliar 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 when Sheldon becomes too demanding. However, Leonard accidentally breaks his glasses and must re-enter the apartment using help with a web-cam from Howard and Raj, 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 or injured on the couch.

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 is 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 he used to and contributing to their work much to their annoyance. 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. Soon enough, 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. 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 to join them; she rejects the offer at first, but reconsiders 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?").

At the bowl, the guys team is called 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 butcher). PMS wins the competition when Sheldon, himself unable to answer the last question, will not 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.

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".

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. Because the men's efforts to move the cumbersome object into the apartment cause Penny to lose a shift at work and much of her day's time, she furiously lashes out at the men for their childlike preoccupation with various genre memorabilia. Leonard, upset that such interests may cost him 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.

But, Sheldon turns the tables in Leonard's favor, when he accuses Penny of hypocrisy, 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. Leonard's hope of a relationship with Penny is then renewed, as is his decision to keep his belongings. However, when a new boyfriend of Penny's named Mike shows up, 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 attacked by the Morlocks. He then wakes up to tell Leonard about this and attacked by the "guys" Leonard hired, who are dressed as Morlocks.

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.

The guys see an attractive woman in Sheldon's office, and they become even more surprised when Sheldon introduces her as his twin sister, Missy. 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. Of course, the men begin awkwardly flirting with 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, believing that he will let him date her; 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 (after they have a little talk in which Missy kicks him in the groin). 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 has 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.

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 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.

Recurring character: Vernee Watson as Althea

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 is convinced the Chinese restaurant is selling "Tangerine chicken" yet is really using oranges, a cheaper citrus fruit.

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" thought 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 mentions the "Schrödinger's cat" experiment, passionately kisses her, 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 is fighting with the owner in Mandarin, and is doing it very badly, disturbing the visitors.

Guest star: James Hong as Chen