Season 1, Episode 1
September 24, 2007
"The Big Bran Hypothesis"
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The Pilot is the only episode in the entire series whose title does not mimic a scientific principle, and was actually the second pilot filmed following the mixed reviews of the Original Pilot. The original Pilot followed a slightly different plot, which involved a more hardcore female lead named Katie. Due to her overly negative attitude, Katie was mostly disliked by the audiences, and therefore re-written as Penny.
Leonard and Sheldon are two brilliant physicists who live together and work together at Caltech. Upon returning home one day, they spot their new neighbor Penny, whom Leonard immediately develops a crush on.
Leonard and Sheldon go to a "High I.Q." sperm bank, but back out while filling out the donation form after Sheldon informs Leonard that "there is no guarantee that our sperm is going to generate high I.Q. offspring", ergo, they are committing genetic fraud.Upon returning to their apartment, the two spot Penny, who is moving into the apartment across the hall from them. Leonard and Sheldon agree that she is an improvement over their old neighbor, Louie/Louise, who, according to Sheldon, was a 200-pound transvestite with a skin condition. Penny is invited over to their apartment and is impressed by their work. Penny is introduced to Sheldon's obsessiveness when she accidentally sits in Sheldon's spot on the couch, as well as to their lifestyle, like their game of Klingon Boggle.
Penny's shower is out of order, so she borrows Leonard and Sheldon's. As she is taking a shower, Howard and Rajesh arrive with a video of Stephen Hawking from 1973, which excited them because it was before he had to use a computerized voice simulator. Penny exits the shower only wearing a towel because she can't get the water to go from tub to shower, and she is introduced to them. Howard, thinking he is a professional ladies' man, hits on Penny while Raj is rendered silent since he can't speak in front of women due to his selective mutism, which was unknown at the time.Penny then asks Leonard a favor, asking him to pick up her TV from her ex-boyfriend, Kurt who lives across town. Due to his crush on Penny, Leonard agrees and drags Sheldon along. During their ride to Kurt's apartment, we discover that Leonard's last girlfriend was a North Korean spy named Joyce Kim. Upon their arrival, they are denied access to the building, but by observing two clever girl scouts get into the building, they quickly slip in behind them. Penny's ex-boyfriend, who turns out to be a tall and very muscular man, refuses to give up the TV, and apparently finds Leonard and Shelson so irritating that for that he steals their pants. They return to their apartment, pants-less and TV-less. Penny feels sorry for Leonard and Sheldon after their incident and decides to make it up to them by paying for their dinner (along with Howard and Raj). While looking at Penny's apartment Sheldon tells Leonard that this thing with Penny isn't over. Leonard tells him that their children will be smart and beautiful, to which Sheldon adds, "Not to mention imaginary." In the car, the episode ends with Sheldon telling Leonard that despite the low chances of him starting a relationship with Penny, in comparison to the other males in the car; he is a "veritable Mac Daddy ".
(Leonard and Sheldon are going upstairs to their apartment after an unsuccessful visit to the "High IQ" sperm bank. They have to use the stairs since the elevator has been broken for at least four years.)
- Sheldon: Are you still mad about the sperm bank?
- Leonard: No.
- Sheldon: You wanna hear an interesting thing about stairs?
- Leonard: Not really.
- Sheldon: If the height of a single step is off as little as 2 millimeters, most people will trip.
- Leonard: I don't care... 2 millimeters? That doesn't seem right.
- Sheldon: No, it's true, I did a series of experiments when I was 12; my father broke his clavicle.
- Leonard: Is that why they sent you to boarding school?
- Sheldon: No, that was a result of my work with lasers.
- (The guys are about to go in their apartment, but they soon realise there is a new neighbor across the hall who has just moved in. She is a young and attractive blonde woman and is reading a magazine.)
- Leonard: New neighbor?
- Sheldon: Evidently.
- Leonard: Significant improvement over the old neighbor.
- Sheldon: 200-pound transvestite with a skin condition?Yes, she is.
- (The woman notices the guys.)
- Woman: Oh, hi.
- Leonard: Hi.
- Sheldon: Hi.
- Leonard: Hi.
- Sheldon: Hi.
- Woman: Hi?
- Leonard: We don't mean to interrupt, we live across the hall.
- Woman: Oh, that's nice.
- Leonard: Oh, w..., a..., w..., we don't live together, we..... We live together, but in, separate, heterosexual bedrooms.
- Woman: Oh, OK, guess I'm your new neighbor. Penny.
- Leonard: Uh, Leonard, Sheldon.
- Penny: Hi.
- Leonard: Hi.
- Sheldon: Hi.
- Penny: Hi.
- Leonard: Hi.
- (There is a short silence.)
- Leonard: Well, uh, oh, welcome to the building.
- Penny: Thank you, maybe we can have coffee sometime.
- Leonard: Oh, great.
- Penny: Great.
- Sheldon: Great.
- Leonard: Great (silence). Well, uh, bye.
- Penny: Bye.
- Sheldon: Bye.
- Leonard: Bye.
- (Penny closes her door.)
- (The guys have decided to invite Penny over for lunch.)
- Penny: So, what do you guys do for fun around here?
- Sheldon: Well, today we tried masturbating for money.
- (Leonard and Sheldon are about to get Penny' s T.V. from her ex-boyfriend, Kurt . They're about to talk to the intercom.)
- Leonard (To Sheldon): Now let me do the talking.
- Kurt (via intercom): Yeah.
- Leonard: (via intercom): Hi, I'm Leonard and this is Sheldon.
- Sheldon (To the intercom.): Hello
- Leonard (To Sheldon muttering a few random words): Ah! Uh, just... (To Kurt via intercom) We're here to pick up Penny's T.V.
- Kurt (via intercom): Get lost.
- Sheldon (To Kurt via intercom.): OK, well, thanks for your time.
- (After being refused entry into the building by Kurt, Sheldon and Leonard manage to get into the building after some girl scouts selling cookies enter and the guys sneak in after them before the door closes. They then arrive at Kurt's door. When the two see what Kurt looks like (a big, strong, and tough guy), they get nervous.
- Kurt: Yeah.
- Leonard: Hi. I'm Leonard and that's Sheldon.
- Sheldon: From the intercom.
- Kurt: How the hell did you get in the building?
- (Leonard is frightened and Lost for words)
- Leonard: Uh...we're scientists.
- (Kurt is just staring at him, Sheldon whispers to Leonard)
- Sheldon: Tell him about our IQ.
- (A short time later, Leonard and Sheldon come out the building without their pants as Kurt stole them from the guys.)
- Sheldon: Leonard.
- Leonard: Yes?
- Sheldon: My mom bought me those pants.
- Leonard: I'm sorry.
- Sheldon: You're gonna have to call her.
(After Penny has just met Howard and Raj, she slumps down on the couch next to Rajesh .)
- Penny: (To Raj): So, you guys work with Leonard and Sheldon at the university?
- (Raj doesn't answer and just continues to eat his food.)
- Penny: I'm sorry? Do you speak English?
- Howard: Oh, he speaks English, he just can't speak to women.
- Penny: Oh. Why?
- Howard: He's kind of a nerd. (starts talking smooth) Juice Box?
Penny: (after seeing Leonard and Sheldon pantsed): I'm so sorry. I really thought if you guys went instead of me, he wouldn't be such an ass.
Leonard: No, it was a valid hypothesis.
Sheldon: That, was a valid hypo--what is happening to you?
(Penny gets her first look at Sheldon's work on his white board.)
Penny: This looks like some serious stuff, Leonard, did you do this?
Sheldon: Actually that’s my work.
Sheldon: Yeah, well, it’s just some quantum mechanics, with a little string theory doodling around the edges. That part there, that’s just a joke, it’s a spoof of the Bourne-Oppenheimer approximation.
Penny: So you’re like, one of those, beautiful mind genius guys.
Penny is a brave new world for Sheldon and Leonard and it’s a fun start to The Big Bang Theory. The show will have to work hard to maintain the delicate balance between geek jokes and real characters. I wish them luck." - The TV Critic's Review
- Title reference: Only episode that had no individual title.
- This episode was not the first pilot, the first one was unaired because it was not approved.
- This episode was watched by 9.52 million people with a rating of 3.7 (adults 18-49). The Pilot is the second most viewed episode in the first season.
- Chuck Lorre's vanity card 
- Episode transcript 
- Sheldon wears briefs and Leonard wears boxers.
- Sheldon reveals that he has 212 friends on Myspace.
- Sheldon has the first line in the series.
- Sheldon has the last line in the episode (except for Howard, who is still singing).
- There were 6 magazines on the table in the high-IQ sperm bank.
- In this episode, Leonard states that the combined IQ of Sheldon and himself is 360.
- In this episode, Sheldon and Leonard are seen drinking bottled orange sodas, which sparked controversy over whether it was beer or not, due to Sheldon's incredibly low alcohol tolerance.
- When the guys first notice Penny, her boombox is playing the song Smile by Lily Allen.
- The scene where Sheldon and Leonard observe Penny through her open apartment door would be repeated as an homage in the series' 100th episode, Season 5's "The Recombination Hypothesis". The characters would even be dressed similarly, right down to Penny wearing the exact same blue top that she wears here.
- Penny's clothes change after her shower, but she didn't have another set with her and it appears she didn't return to her apartment since Raj and Howard were there.
- Sheldon refers to a window in the kitchen, which is there, but is usually obscured by a whiteboard. This implies apartment 4A is along a corner of the building.
- Darth Vader Shampoo and Luke Skywalker Shampoo actually exist, but there isn't a conditioner, so it's likley Leonard reuses Darth Vader as shampoo and Luke Skywalker as the conditioner.
- While Penny has a pleasant outgoing personality, Katie from the unaired pilot opening was meaner and mocked them. Penny does have a temper and some of these personality traits came over from the woman in the first pilot when Penny is really mad.
- The family names of Howard (Wolowitz) and Rajesh (Koothrappali) are revealed in this episode, while Sheldon's (Cooper) and Leonard's (Hofstadter) are not.
- When aired in syndication, the sperm bank scene is not shown, likely because it contrasts with the nearly asexual Sheldon that would be established in later seaons' episodes.
- The experiment discussed by Sheldon at the very beginning of the episode is a quantum version of the well-known "double-slit experiment."
- This is the only show where Raj was seen with a hat.
- The song that Howard sings to Penny at the end is "Baby, Don't Get Hooked on Me" by Mac Davis.
- This Pilot was rewritten and filmed several times.
- Sheldon: Yeah, well, it’s just some quantum mechanics with a little string theory doodling around the edges. That part there's just a joke. It's a spoof of the Born-Oppenheimer approximation.
- Sheldon's dialogue is incompatible with the work presented on the whiteboard. This one (left) describes the decay of the top quark. For more detail, see the blog here.
- Sheldon's dialogue was meant to accompany the whiteboard seen in the Unaired Pilot (right).