Penny struggles with Leonard's confession that he kissed another woman as they prepare to get married in Las Vegas. Meanwhile, Sheldon doesn't know what to do after Amy pushes pause on their relationship.
Leonard and Penny try to work through their marital issues. Meanwhile, Sheldon films a special episode of "Fun with Flags" after his breakup with Amy, and Bernadette feels guilty about withholding a secret from Penny.
Wil Wheaton interviews Sheldon for a Star Trek documentary where he reveals the ring and his plan to propose to Amy. Bernadette wants to start renovating the house and the topic of starting a family together with Howard is brought up.
On Thanksgiving, Sheldon and Amy try to spend time together as friends and go to an aquarium, Howard reluctantly agrees to volunteer at the soup kitchen with Bernadette, Raj and Emily, and Penny discovers that Leonard knows more about her than she thought.
Sheldon has a revelation when he realizes why a song was stuck in his head. Meanwhile, Amy invites Dave over for dinner despite their previously awkward date, and Howard and Raj become obsessed with a fan of their band.
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 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.
Amy convinces Sheldon to celebrate his birthday for the first time since his childhood. The group plans a birthday party for him, convincing celebrated Batman actor Adam West. But when Sheldon enters the party he is overcome with anxiety.
Sheldon works up a contract to split their portion of their guidance system invention royalties, but Bernadette does not like how it's done. Raj must again decide between Emily and Claire when they both contact him.
Leonard and Penny try to have a wedding ceremony everyone can attend, but it creates serious conflict among Sheldon's mother and Leonard's divorced parents. Howard's invention draws the attention of the Air Force.