What happens when hyperintelligent roommates Sheldon and Leonard meet Penny, a free-spirited beauty moving in next door, and realize they know next to nothing about life outside of the lab. Rounding out the crew are the smarmy Wolowitz, who thinks he's as sexy as he is brainy, and Koothrappali, who suffers from an inability to speak in the presence of a woman.
After Leonard's father, Alfred and Sheldon's mother, Mary share an evening together, everyone deals with an awkward morning the following day. Also, Penny's family arrives for the ceremony, including her anxiety-ridden mother, Susan, and her drug dealing brother, Randall.
The Air Force shows interest in the guidance system and Penny lets it slip at work that Bernadette is expecting.
Sheldon can't keep up with the late nights Howard and Leonard are putting in, Raj spends the day with a pregnant Bernadette and Penny and Amy attend a faculty party.
When Amy's apartment floods, she proposes a "cohabitation experiment" with Sheldon. Also, Howard and Bernadette are upset when Koothrappali learns the gender of the baby before they do.
Leonard and Penny must separate a quarreling Sheldon and Amy when their cohabitation does not go as planned, and Sheldon threatens to break off their relationship when Amy refuses to adhere to the bathroom schedule. Also, Howard and Bernadette find unexpected guests at their house when they decide to stay home from a planned vacation.
Penny is surprised to learn she has fans of her performance in "Serial Ape-ist" when she attends Van Nuys Comic Con with Leonard. Also, Amy hosts a Sheldon-style brunch at Penny’s apartment, and Wolowitz makes an impulsive purchase when he starts stressing out about the baby on the way.
In order to continue living with Sheldon, Amy lies to him about repairs being made to her apartment and they host an "on location" episode of "Fun With Flags" in Penny’s apartment. Also, Leonard lets Penny decorate Sheldon’s old room after he learns that she has been secretly moving Leonard’s collectibles into storage.
After a successful experiment combining their genes, Sheldon will stop at nothing to convince Amy they should procreate. Also, Koothrappali is embarrassed to tell the gang what the new woman he’s dating does for a living.
When Bert, a Caltech geologist, wins the MacArthur Genius fellowship, Sheldon is overcome with jealousy. Also, Wolowitz finds an old remote control Stephen Hawking action figure he invented.
Sheldon and Leonard must divide up their possessions and remove Penny's things from 4B.
Amy's desire to have an intimate night with Sheldon on her birthday is interrupted by Bernadette going into labor.
Sheldon and Amy visit Leonard and Penny for the first time since the holidays and share details about their terrible trip to Texas.
Leonard and Penny may need a Relationship Agreement, and Sheldon volunteers to prepare one.
Sheldon uses a machine to help him identify other people's emotions, and Raj tries to understand his difficulties with women.
Leonard and Wolowitz try to distract Sheldon when he slows the progress of their guidance system. Also, Penny and Amy take Bernadette out for the night, leaving Raj and Stuart to care for baby Halley.
Sheldon and Amy have their first fight since moving in together and Koothrappali is shocked at the realities of financial independence when he stops receiving support from his father.
The guys' annual trip to Comic Con is in doubt when Raj can't afford to go.
Raj searches for a cheaper living arrangement.
Leonard, Penny and Raj adjust to their new living arrangement and Sheldon takes an interest in Amy's work.
Sheldon pushes himself to the limit when he collaborates on projects with Leonard and Wolowitz as well as Amy on the same day. Also, Bernadette is nearing the end of her maternity leave and must return to work.
Howard and Bernadette have a hard time leaving Halley in day care when Bernadette returns to work. Also, Bert introduces the gang to his new girlfriend, Rebecca.
Leonard isn’t thrilled when Penny considers taking a new job offered by her ex-boyfriend, Zack.
Amy is offered a Fellowship at Princeton, Raj makes new living arrangements, and the gyroscope project is spinning out of control!
The gang is concerned when Sheldon’s former admirer, Dr. Ramona Nowitzki, resurfaces while Amy is away at Princeton.