Penny runs into her ex-boyfriend Zack, who has gotten rich from selling his company and married a similarly dim-witted woman named Marissa. Meeting with Leonard and Penny, they say Zack is infertile and ask to pay Leonard as a sperm donor so they can have a baby. Leonard is flattered, but Penny finds it creepy. Leonard says that since Penny decided she did not want to have children, he should be allowed to help another couple with this issue. Sheldon advises him to think about the emotional toll this could have on him. Leonard says he will not do it if Penny objects; Penny says she will support whatever decision Leonard makes. Meanwhile, Raj tries to get Anu back but she points out he just wanted to be married like his friends. Raj points out that Anu is similarly looking for a quick marriage. He admits to Anu that he really likes her and they decide to start their relationship as just dating.
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.
Howard dresses as Sheldon for Halloween at work, hurting Sheldon's feelings. When Howard does not apologize, Amy and Sheldon dress as Bernadette and Howard at Penny and Leonard's party, angering Bernadette. Sheldon and Bernadette later bond over their painful childhood memories. Leonard is upset that Penny does not seem to remember that their first kiss was on Halloween. Penny later admits she does remember, but does not like to think of their first kiss as an unromantic drunken hookup. She likes to imagine their first kiss was one they shared on Leonard's birthday, which pleases him.
Kunal Nayyar has the flick Think Like a Dog in post-production and the upcoming Trolls World Tour in pre-production. Nayyar provides the voice of Guy Diamond in the movie series. Aside from that, however, the former Raj Koothrappali is looking forward to some much needed family time. "I'm going to go to India, spend more time with my parents," he told Parade in late April. "I left my country when I was 18, so it's important for me to go home and spend time with mom and dad, so that's the main focus right now.
Leonard's mother Beverly visits, and he is thrilled when she takes an interest in his work. However, when he learns that she is doing research on him for a new parenting book, he is furious. He prepares to tell her how she let him down over the years, but instead chooses to accept and forgive her, and forgive himself for holding onto that resentment for so long. Beverley is touched that he forgave her and they share a genuine hug. Meanwhile, Anu gets a job offer in London. Raj prepares to fly out and propose to her, ready to follow her if she accepts. Howard stops Raj at the airport, saying there is a better match out there for him, convincing Raj to return home. Stuart and Denise are bothered by her creepy roommate Mitch, while Bernadette and Howard are tired of Denise constantly staying overnight with Stuart at their house. Stuart says he is ready to move in with Denise, and they say they love each other while convincing Mitch to move out.
Kaley Cuoco as Penny:[44] An aspiring actress from Omaha, Nebraska. Penny moves in across the hall from Sheldon and Leonard. She waits tables and occasionally tends the bar at the Cheesecake Factory. After giving up hope of becoming a successful actress, Penny becomes a pharmaceutical sales representative. Penny becomes friends with Bernadette and Amy, and they often hang out in each other's apartments. Penny and Leonard form a relationship and eventually marry.

After Sheldon has a food truck moved away from the building, Leonard and Penny are shocked to find he is the leader and sole member of the Tenants' Association for the building. Amy doesn't want to be caught in the middle but secretly suggests they rally support from the other tenants to vote Sheldon out. Nobody else supports Leonard so Amy tells them Sheldon was never added to the lease when he moved across the hall with her. Sheldon fires back with his own legal technicality so Amy is forced to support Leonard. She makes Sheldon happy by saying he can be Leonard's official opposition, making Leonard believe he made a mistake. Meanwhile, Howard and Raj find a drone. Raj returns it to the cute owner Cynthia and gets her number. Unfortunately, Cynthia watches footage of Raj on the drone and is immediately turned off.
The Big Bang Theory is an American comedy television series created and executively produced by Chuck Lorre and Bill Prady. Like the name of the series itself (with the exception of the first episode "Pilot"), episode titles of The Big Bang Theory always start with "The" and resemble the name of a scientific principle, theory or experiment, whimsically referencing a plot point or quirk in that episode.

Melissa Rauch as Bernadette Rostenkowski-Wolowitz (recurring season 3, starring seasons 4–12):[51] A young woman who initially is a co-worker at The Cheesecake Factory with Penny to pay her way through graduate school, where she is studying microbiology. Bernadette is introduced to Howard by Penny; at first, they do not get along, apparently having nothing in common. They date and later get engaged and married. Although generally a sweet and good-natured person, Bernadette has a short fuse and can be vindictive and lash out when provoked.
Simon Helberg doesn't have any projects listed in development as of now, but he has had a burgeoning movie career. He was nominated for a Golden Globe for Florence Foster Jenkins and also appeared in Paint It Black. Maybe he's holding out hope for a spinoff for his Howard Wolowitz? When THR asked creator Chuck Lorre if he was gestating on ideas for any Big Bang character spinoffs, he coyly responded, "You can't help but gestate a little..."
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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