Penny gets more than she bargained for when Leonard agrees to meet with a psychiatrist on her behalf to make a sale. Meanwhile, Sheldon and Raj collaborate on an astronomical discovery, and Howard and Bernadette's happiness is short-lived after Stuart moves out.
Sheldon is thrilled when his Meemaw comes to visit, but his excitement quickly fades when she butts heads with Amy. Meanwhile, Raj meets a girl at the comic book store who makes him question his relationship with Emily.
Sheldon and Amy host a Valentine's Day episode of "Fun with Flags", Leonard and Penny grapple with getting older, Howard and Bernadette find a surprise in their hot tub and Raj is torn between Emily and Claire.
Leonard, Sheldon, and Howard run into problems when they file for a patent for their infinite persistence gyroscope. Meanwhile, the girls give Raj dating advice when Emily reaches out to him after their breakup.
Amy is shocked at a revelation from Sheldon after she buys him a new laptop. Meanwhile, Raj sells out Leonard and Howard after they lie to their wives in order to attend an early screening of a movie.
Secrets are revealed when Amy and Sheldon join Penny and Leonard on a weekend cabin trip. Raj is more excited about Howard's and Bernadette's impending parenthood than they are.
Sheldon and Bernadette spend the evening together while the rest of the gang go to a wine tasting party, but things get uncomfortable for Raj when Penny's ex-boyfriend Zack shows up and takes an interest in Claire.
Sheldon hires Stuart to spend the day with Amy when he would rather go to the movies. Meanwhile, Penny struggles to make a connection with Leonard's mother when she comes to visit.
Leonard and Penny try to have a wedding ceremony that everyone can attend, but it creates serious conflict among Sheldon's mother and Leonard's divorced parents. Meanwhile, Howard and Raj become convinced the government is out to get them.
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.