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.
Leonard angers the University and the Physics Community when he gives an embarrassing radio interview. Bernadette and Amy talk about their success. After the embarrassing radio interview Penny comforts her husband.
Leonard is devastated by the new friendly relation developing between his mother and Penny. Meanwhile Sheldon takes a detour with Raj and Howard to explore the old belongings of Howard's Science Models.
Howard and Leonard become upset after they discover that Sheldon has been working with the military without telling them; Bernadette asks Raj to investigate her suspicions that Ruchi is attempting to take her job away from her.
Sheldon and Amy try to eliminate stress from wedding planning by applying math to the process ending up at city hall. Also, Raj "breaks up" with Howard after realizing his best friend is actually hurting his confidence.
Sheldon and Wolowitz plan birthday celebrations for Amy and Halley, respectively. But later things go wrong. Amy and Sheldon get sick during the party and Raj help Wolowitz plan for the but later on things went wrong when they quarrel which put their friendship in jeopardy but later they find ways to get along.