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.
Amy neglects to tell Sheldon her apartment has been repaired. Leonard and Penny discuss what is to be done with Sheldon's room, but Leonard becomes more upset that Penny's been storing away his collectibles.
After spending the holidays apart, the gang gets together to hear about Sheldon's and Amy's trip to his mother's, Leonard's and Penny's Christmas tree adventure, and Howard's and Bernadette's first days as parents.
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.