Sheldon cannot find the perfect wedding date and begins talking in his sleep, adopting the persona of a laid-back person who goes with the flow. Amy and Penny believe that his subconscious mind is trying to tell him to be more relaxed; his attempt to be so while wearing flip-flops ends in disaster. He decides to leave all the wedding planning to Amy while he wears a Star Trek uniform underneath his tuxedo. Meanwhile, Raj and Stuart both become interested in Bernadette's new coworker Ruchi. While she likes them both as friends, she does not want a relationship with anyone.
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. 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.
In November 2016, it was reported that CBS was in negotiations to create a spin-off of The Big Bang Theory centered on Sheldon as a young boy. The prequel series, described as "a Malcolm in the Middle-esque single-camera family comedy" would be executive-produced by Lorre and Molaro, with Prady expected to be involved in some capacity, and intended to air in the 2017–18 season alongside The Big Bang Theory. The initial idea for the series came from Parsons, who passed it along to The Big Bang Theory producers. In early March 2017, Iain Armitage was cast as the younger Sheldon, as well as Zoe Perry as his mother, Mary Cooper. Perry is the real-life daughter of Laurie Metcalf, who portrays Mary Cooper on The Big Bang Theory.
Kevin Sussman as Stuart Bloom (recurring seasons 2–5, 7, starring seasons 6, 8–12): A mild-mannered, under-confident owner of a comic book store. A competent artist, Stuart is a graduate of the prestigious Rhode Island School of Design, and though he is socially awkward he possesses slightly better social skills. Stuart implies he is in financial trouble and that the comic book store now also is his home. He is later invited to join the guys' group while Howard is in space. Stuart gets a new job caring for Howard's mother later. After Mrs. Wolowitz's death, Stuart continues to live in her home, along with Howard and Bernadette, until he finds a place of his own.
Kunal Nayyar as Rajesh Koothrappali: A particle astrophysicist originally from New Delhi, India. Raj is initially unable to talk to women unless he drinks alcohol. Raj also has very feminine tastes and often takes on a stereotypical female role in his friendship with Howard as well as in the group of four men. Raj later dates Lucy (Kate Micucci), who also suffers from social anxiety, but it eventually ends; he later speaks to Penny without alcohol and thus overcomes his selective mutism. He begins dating Emily and their relationship later becomes exclusive. Raj also has a Yorkshire Terrier named Cinnamon.
With Leonard, Howard, Raj, and Amy accomplishing so much on their respective projects, Sheldon is forced to admit he has nothing important upon which to work. He makes Amy leave the apartment for a few days so he can focus, but cannot come up with any ideas and calls his mother as a distraction. Leonard and Amy have fun recreating experiments from when they were growing up, boring Penny, so she eats with Sheldon as he mulls over his scientific studies. Penny helps him realize that his study of dark matter is his rebound science from string theory, which Sheldon admits he never truly disregarded, but explaining string theory to her inspires Sheldon, helping him discover a potential breakthrough in the field. Meanwhile Howard is too busy with his family to be in the band with Raj, so Raj brings in Bert. But, when Howard annoys Bernadette by writing an astronaut-themed musical while she is on bed-rest, she makes him rejoin the band. The three are poorly received at a Bar mitzvah after singing Bert's original song about the boulder from Indiana Jones .
Sara Gilbert as Leslie Winkle (recurring season 1, starring season 2, guest seasons 3, 9): A physicist who works in the same lab as Leonard. In appearance, she is essentially Leonard's female counterpart and has conflicting scientific theories with Sheldon. Leslie has casual sex with Leonard and later Howard. Gilbert was promoted to a main cast member during the second season but resumed guest star status because producers could not come up with enough material for the character. Gilbert returned to The Big Bang Theory for its 200th episode.
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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.
Sheldon's childhood friend from Texas, Tam, is visiting the university with his son. Sheldon does not want to see him and the others wonder why Sheldon has never mentioned Tam. Tam is unaware Sheldon is angry with him, thinking they just naturally drifted apart. Sheldon's brother George does not know the reason either. Sheldon eventually reveals Tam promised he would move to California with Sheldon, but then got a girlfriend and stayed with her, leaving Sheldon feeling abandoned. Tam and Sheldon make up and Sheldon has Leonard take Tam to the airport. Meanwhile, Penny and Bernadette want to get to know Anu, impressed by her connections in the hotel industry. They tell her embarrassing things about Raj and wonder if they can go out with her again after Raj confronts them.
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.