Leonard and Penny renew their vows, this time inviting their friends and family for wedding party; they also learn that marriage is about compromise, and figure out how to support each other. Sheldon and Amy experiment to take their relationship to the next level, and after an emotional struggle they move in together in Penny's old apartment. Howard and Bernadette realise that being a parent is a full-time job, but with the help of their friends they manage to find joy in raising their child. When his friends tell him he's extremely spoiled, Raj decides to stop taking his father's money and learn how to make ends meet.
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 the guys are building 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 and must resort to ingesting energy drinks to stay awake. Elsewhere, Raj spends the day with a pregnant Bernadette, while 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. Meanwhile, Koothrappali and Stuart fight to be the most helpful during Bernadette's final weeks of pregnancy.
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.
When Penny feels that Leonard is taking her for granted, she decides to go on a spa weekend with Amy. Also, Wolowitz and Koothrappali try to fix a squeaky floorboard in the baby's room.
Sheldon beta-tests an MIT invention after hearing it can help him read people's emotions. Also, Leonard can't hide his feelings when Penny announces her brother is coming to town, and Raj invites his ex-girlfriends over to give him dating advice.
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 and an unexpected replacement is considered.
Raj searches for a cheaper living arrangement and it creates unexpected conflict among everyone.
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.
After celebrating the completion of the top-secret Air Force project, Leonard, Sheldon and Howard get an unwelcome surprise; Amy receives an offer for a summer position at Princeton as a visiting researcher.
The gang is concerned when Sheldon’s former admirer, Dr. Ramona Nowitzki, resurfaces while Amy is away at Princeton.