One of the recurring plot lines is the relationship between Leonard and Penny. Leonard becomes attracted to Penny in the pilot episode and his need to do favors for her is a frequent point of humor in the first season. Meanwhile, Penny dates a series of muscular, attractive, unintelligent, and insensitive jocks. Their first long-term relationship begins when Leonard returns from a three-month expedition to the North Pole in the season 3 premiere. However, when Leonard tells Penny that he loves her, she realizes she cannot say it back. Both Leonard and Penny go on to date other people; most notably with Leonard dating Raj's sister Priya for much of season 4. This relationship is jeopardized when Leonard comes to falsely believe that Raj has slept with Penny, and ultimately ends when Priya sleeps with a former boyfriend in "The Good Guy Fluctuation".
The show originally centered on five characters living in Pasadena, California: Leonard Hofstadter and Sheldon Cooper, both physicists at Caltech, who share an apartment; Penny, a waitress and aspiring actress who lives across the hall; and Leonard and Sheldon's similarly geeky and socially awkward friends and co-workers, aerospace engineer Howard Wolowitz and astrophysicist Raj Koothrappali.[4][5] Over time, supporting characters were promoted to starring roles, including neuroscientist Amy Farrah Fowler, microbiologist Bernadette Rostenkowski, physicist Leslie Winkle and comic book store owner Stuart Bloom.
Producers, writers and others, including recurring guest star Wil Wheaton, joined the festivities on set and — hardly a spoiler alert — more laughing, crying and hugging ensued. Lorre embraced a happy Parsons, then moved on to each cast member as if it were a reception line. Galecki shared long, emotional hugs with his colleagues. After trying to keep their emotions under control for so long, it was finally time to just let go.
After Neil Gaiman tweets about Stuart's comic book store, it becomes very popular, and the crowds upset Sheldon. Stuart hires an assistant manager named Denise and she impresses Sheldon with her comic recommendations. Amy goes to her for information on comics to connect to Sheldon but when she tries to tell him, he says he's talked enough about comics with Denise. The store's popularity causes Stuart to be unable to babysit for Howard and Bernadette's date night. They try to have another at home, but Stuart shows up so they hurriedly go out. Meanwhile, as Raj sets up a telescope, Penny sees a new comet. Raj takes sole credit for the discovery, angering Penny. Leonard tries to defend her but feels sorry for hurting Raj's career. Penny confronts Raj herself and he agrees to share credit after Penny doesn't give in like Leonard.

Although the original pilot was not picked up, its creators were given an opportunity to retool it and produce a second pilot. They brought in the remaining cast and retooled the show to its final format. Katie was replaced by Penny (Kaley Cuoco). The original unaired pilot has never been officially released, but it has circulated on the Internet.[citation needed] On the evolution of the show, Chuck Lorre said, "We did the 'Big Bang Pilot' about two and a half years ago, and it sucked ... but there were two remarkable things that worked perfectly, and that was Johnny and Jim. We rewrote the thing entirely and then we were blessed with Kaley and Simon and Kunal." As to whether the world will ever see the original pilot on a future DVD release, Lorre said, "Wow, that would be something. We will see. Show your failures..."[9]
Jim Parsons as Sheldon Cooper:[43] Originally from Galveston, Texas, Sheldon was a child prodigy with an eidetic memory who began college at the age of eleven, and earned a PhD at age sixteen. He is a theoretical physicist researching quantum mechanics and string theory, and despite his IQ of 187, he finds many routine aspects of social situations difficult to grasp. He is determined to have his own way, continually boasts of his intelligence, and has an extremely ritualized way of living. Despite these quirks, he begins a relationship with Amy Farrah Fowler, and they eventually marry.

Sheldon has been writing to the reclusive scientist Dr. Wolcott (Peter MacNicol) who invites Sheldon to visit him in his completely isolated mountain cabin. The women quickly ask Leonard, Raj and Howard to tag along with him, making this his bachelor party, despite Sheldon's assertions that it shouldn't be called such. Wolcott has a brilliant mind but his behavior is very strange to the guys. He tells Sheldon he cut off all contact with others to focus on science. Though Sheldon enjoys the science, he decides that he doesn't want to live like Wolcott because of his friends and Amy. The guys leave to avoid Wolcott, whose theories about time make him doubt whether the guys were ever there. Meanwhile, Penny and Bernadette throw Amy a quilting bee for her bachelorette party, which she quickly finds boring. When Amy tells them she wants a night full of bad decisions as she earlier implied, they go to a bar where Amy drinks a few shots and passes out in twelve minutes. When she wakes up back at the apartment hungover and disappointed, the women lie and reassure her she had a wild time, involving her drunkenly riverdancing.
After Sheldon reminds the guys about the time they mined Bitcoin without him, the guys go on a quest to find their suddenly valuable missing cryptocurrency. They realize the currency is on a laptop which Leonard gave to Penny when they dating, which she then gave to her ex-boyfriend Zack. After Leonard and Penny go to Zack's apartment, he shows them a seven-year-old video that Penny recorded on the laptop soon after she broke up with Leonard. Leonard is touched by the video which shows a drunk Penny lamenting the break-up of her relationship with him. 
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