Sheldon's trip comes to an end when he is robbed, Penny interviews for a job at Bernadette's company, and Howard is weirded out by his mother's relationship with Stuart.
Howard becomes nervous after NASA asks him to throw out the first pitch at a Los Angeles Angels game. Meanwhile, Leonard and Penny try to prove Sheldon wrong when he says that he and Amy have a 'superior' relationship.
Raj's honesty about his past comes back to haunt him after Emily takes a dislike to Penny. Meanwhile, the guys consider investing in the comic book store.
Sheldon and Raj test whether they could survive a dark-matter research expedition in a salt mine by simulating the conditions in a steam tunnel. Meanwhile, Leonard and Penny seek advice from Howard and Bernadette when they argue about money.
The guys befriend one of Penny's clients when he shows up at her apartment. Meanwhile, Bernadette gets upset when Amy torpedoes an article about her for a local magazine.
The girls decide to re-enact their high school proms with the guys. While Sheldon feels under pressure to commit to his relationship with Amy, Howard worries about who Stuart will bring as his date.
While Sheldon retires "Fun With Flags", Leonard, Howard and Raj look for something significant in a dead professor's research, and Bernadette finds out what her colleagues really think of her.
Amy hosts a traditional Victorian Christmas Eve dinner as Raj's father comes to visit, while the guys try to shoo out a bird they let into the university clean room.
When Sheldon becomes stuck in a rut with his work, the gang try to help him by raising his stress levels. Howard teases Raj with a game called "Emily or Cinnamon."
Howard receives some shocking news as everyone prepares for the re-opening of the comic book store. Meanwhile, Penny tries to teach Sheldon how to "let it go," but can't follow her own advice when she learns something infuriating about Amy.
Sheldon and Penny agree to participate in an unusual experiment. Leonard, Amy, Raj and Emily spend the evening trying to escape a room with a "zombie". Howard and Bernadette run into trouble at the airport after returning from Mrs. Wolowitz's funeral.
A trip to the pet store ends in a fight after Amy learns Sheldon has applied for a one-way mission to colonize Mars. Meanwhile, Leonard surprises Penny with a very "adult" purchase, and Raj gets into trouble when he snoops around Emily's apartment.
Tension arises when a magazine fails to mention Leonard in an article about the paper he co-wrote with Sheldon, and a blackout at Mrs. Wolowitz's home leads to a final family dinner in her honor.
When Leonard and Sheldon are invited to speak at UC Berkeley, they take a detour to try and meet one of their idols. Meanwhile, while cleaning out Mrs. Wolowitz's house, a ping pong battle ensues when Howard and Bernadette argue over the fate of his TARDIS.
When Raj is asked to create a message on the off chance a NASA mission discovers alien life, the guys fight over what he should do, while Penny contemplates returning to acting.
Howard questions his engineering abilities when he and Sheldon can't get a toy drone to fly. A cancelled flight nearly prevents Leonard from giving the commencement address at his former high school. Raj pits his parents against each other when his father cuts him off financially.
As Sheldon presses Leonard and Penny to set a wedding date, he encounters major changes in his relationship with Amy. Meanwhile, Raj wonders if he should break up with Emily, and Howard and Bernadette struggle to ask Stuart to move out.
