As Leonard and Penny attempt to hold another wedding ceremony everyone can attend, they have to deal with Penny's family arriving and Leonard's father spending the night with Sheldon's mother.
Bernadette confesses to Raj that she isn't sure if she's ready to be a mother, Penny accompanies Amy to a university mixer, and Sheldon goes to unusual extremes to assist Howard with his military project.
When Amy must vacate her apartment for repairs, living arrangements are made with Leonard, Sheldon, and Penny. Howard and Bernadette don't want to know the sex of their baby, but Raj finds out.
Sheldon and Amy go through a period of adjustment with their new living arrangement. Howard and Bernadette are forced to vacation at home when she falls ill and keep it a secret, unaware Raj and Stuart planned to use their hot tub.
Sheldon and Amy host a brunch. Penny is surprised by her popularity when her movie, "Serial Ape-Ist" is featured at a Van Nuys comic book convention. Howard starts to freak out about parenthood after feeling the baby kick for the first time.
A successful gene experimentation leaves Sheldon excited to procreate with Amy, and he pulls out all stops to get her on board. Meanwhile, Raj takes an interest in the cleaning lady at the university.
Dividing up their possessions becomes an all-out war between Leonard and Sheldon. Stuart asks to live with Howard and Bernadette and tries to supplant Raj's efforts to help with the new baby.
After spending the holidays apart, the gang get together to hear about Sheldon's and Amy's trip to Texas, Leonard's and Penny's Christmas tree adventure, and Howard's and Bernadette's first days as parents.
Leonard and Penny ask for Sheldon's and Amy's help on putting the romance back in their marriage. Howard and Raj go to extreme lengths to stop the floor in Halley's bedroom from making noise.
Penny wants to help her brother get a job with her company. Sheldon learns of a machine that will help him understand human emotions. Raj reunites some of his old girlfriends to figure out why he is still single.
Leonard and Howard's attempt to keep Sheldon away from their Air Force project by sending him on a train trip backfires. Penny and Amy take Bernadette for her first night out since having the baby.
Sheldon and Amy have their first fight since moving in together. Meanwhile Raj is shocked at the reality of financial independence when he stops receiving support from his father.
Sheldon reaches his limit when he works on projects with Leonard, Howard and Amy in one day. Bernadette has doubts about leaving Halley when she goes back to work.
Leonard is less than thrilled when Penny considers accepting a job offer from her ex-boyfriend, Zack. Sheldon worries that he hasn't reached his full potential soon enough.
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.
