Unaired Pilot

The Big Bang Theory's Unaired Pilot, refers to the original pilot made for the 2006-2007 season that was not picked up by the network, thus it was never aired. Only Johnny Galecki and Jim Parsons were in the cast, and the female lead, Katie, was envisioned as "a street-hardened, tough-as-nails woman with a vulnerable interior". Katie was played by actress Amanda Walsh. They also had a female friend called Gilda (played by Iris Bahr ). The show's original theme music was also different, using Thomas Dolby's hit "She Blinded me with Science"

Summary
Leonard and Sheldon encounter a young woman named Katie crying on the curb, having been left homeless by a break-up with her boyfriend. Leonard, despite Sheldon's objections, invites her to dinner and then to stay a few days at their apartment. She is abrasive, sexy, hard, and sarcastic, but she softens a little when she discovers that the guys aren't trying to get something from her. In the end, she decides she'll crash at their place for awhile.

Extended Plot
The episode opens with Leonard sitting in the sperm bank waiting for Sheldon to finish his deposits, which he eventually finished using a magazine of women with big butts (his fetish). At the desk, after annoyingly finishing the receptionist's crossword puzzle, they are told that they won't be paid in cash but with cheques. On their way from the sperm bank, the duo happen across a crying girl called Kate, who is sitting on the street after a bad break up with her boyfriend has left her homeless. Leonard invites her to the restaurant, to Sheldon's dismay. When Sheldon can't figure out a good excuse, Leonard invites her to move into their flat.

Whilst Katie is showering, a nerdy friend of Sheldon and Leonard called Gilda shows up at the flat. Gilda, who is shown to have the hots for Leonard who fantasizes about marrying him and having kids, feels threatened by Katie's presence but relaxes after Katie says she's not interested. Katie goes out drinking and arrives back at the flat late at night, waking Leonard by smashing a lamp. Katie starts ranting to Leonard about her life and why it sucks, using the whiteboard, drawing colorful diagrams explain her point. Leonard upsets her by suggesting her problems weren't caused by her mother, boyfriend etc but by her making stupid decisions, as a result, she moves out. Katie comes back at the end of the episode explaining that staying there is her only rent free choice. She takes Leonard, Sheldon and Gilda out dancing and they show their awkward dance moves.

Differences
While retaining portions of the pilot's plot, jokes, settings and characters, the Unaired Pilot has several key differences from the Broadcast Pilot, these include:
 * The characters Howard, Rajesh and Penny do not appear. The Penny-type character is known as Katie.
 * Leonard and Sheldon have a geeky friend, colleague and love interest Gilda.
 * Sheldon is not asexual as he had slept with Gilda and has a fetish for women with large hind quarters (butts). He has also seen seven women naked, some of them aren't even relatives.
 * Sheldon doesn't have a spot in his home rather a table at the Bombay Palace.
 * The set of the apartment is different and there is no broken elevator or indication that the apartment building has an elevator. The lighting in the apartment is also dimmer.
 * Sheldon likes to dance.
 * Katie mocking and mean streak carried over to the regular series when Penny gets real mad.

Trivia

 * Katie (Amanda Walsh) was cut from the series due to an audience survey. She was criticised for being too mean towards Leonard and Sheldon.
 * Positive reviews came in about the two guys (Leonard and Sheldon), so Chuck Lorre decided to add two more guys (Howard and Rajesh) into the mix.
 * This episode makes numerous references to a sexually active Sheldon, as opposed to the largely asexual Sheldon of later episodes: Most notably Gilda and Sheldon admit to having previously had sex at a Star Trek convention. Sheldon also apparently has a fetish for women with large rears, and bring a magazine of such women to the sperm bank for arousal.
 * Katie works at a beauty salon. However, when the character is changed into Penny, she is a waitress at The Cheesecake Factory.
 * Surprisingly, the only character other than Sheldon and Leonard to appear in the Broadcast Pilot is Althea, the nurse at the sperm bank. Her character has made several guest appearances on the show since. Her hair is different here than it was in the aired pilot.
 * Sheldon's stated reason for his preferred table at the restaurant would later be adapted for the reasons for his spot.
 * Leonard wielding his lightsaber upon hearing a noise late at night would later be used in The Big Bran Hypothesis.
 * The opening scene at the sperm bank plays out largely as it did in the Pilot, with two significant differences: In the unaired pilot, Leonard and Sheldon are already at the sperm bank, instead of just walking in. Secondly, instead of changing their mind and leaving, Sheldon has already filled his sperm cup, and is paid (after threatening to take back the cup of sperm upon being told he'll be paid by cheque, rather than cash).