Leonard and Amy end up taking an unexpected road trip to Arizona to go get Sheldon. Penny has an interview for a job at Bernadette's company. Stuart's relationship with Mrs. Wolowitz freaks out Howard.
After Sheldon is forced to teach a class, Howard manages to surprise everybody by taking it. Amy plays both Penny and Bernadette as the tension between them increases in regards to Penny's training for her new job at Bernadette's company.
Wolowitz feels really nervous when NASA asks him to throw out the first pitch at a Los Angeles Angels game. Leonard and Penny attempt to show Sheldon he is wrong after he says that he and Amy are a "superior couple."
Penny receives the cold shoulder from Raj's girlfriend after he tells her some information about their past history. The boys think about buying into Stuart's comic-book emporium.
After the guys decide to have a physicist's retreat, the girls head to Las Vegas for the weekend.
Penny receives a company car from the pharmaceutical company where she's now employed but has trouble coming up with the money to pay back Leonard for the one he bought for her. Meanwhile, Sheldon and Raj look at a potential research grant.
Songs: Merle Travis -- Dark as a Dungeon
At her new job, Penny has a client who develops a crush on her. It turns out that he is a middle-aged version of Howard Wolowitz. Amy and Bernadette argue over a photo shoot of female scientists.
Amy, Bernadette, and Penny decide to re-enact their high school proms with their respective fellows. Raj, Emily, and Stuart are included too.
Sheldon panics when Leonard goes into the hospital for an operation. Meanwhile, Raj learns that his parents have gotten a divorce.
Sheldon retires his Fun With Flags You Tube series, while Leonard, Howard, and Raj look through the papers of a deceased colleague and Bernadette discovers what her boss really thinks of her.
Amy fixes a traditional Victorian Era Christmas Eve dinner whose guest list includes Raj's newly separated father, visiting from India.
Songs: Covered by Bernadette -- Here Comes Santa Claus, Covered by Leonard, Howard, and Raj -- Jingle Bells
Raj is concerned over whether a probe he designed will work in outer space. Penny and Amy coerce Leonard and Sheldon into taking them shopping.
The gang tries to help Sheldon when he becomes stuck in a rut at work by increasing his anxiety level. Howard teases Raj with a new game called "Cinnamon or Emily?"
Songs: Johnny Galecki, Kaley Cuoco -- Soft Kitty
Leonard and Sheldon publish a paper which is widely accepted except for comments from an internet troll. Meanwhile, the girls reminisce about embarassing incidents from their past which include internet videos featuring Penny and Bernadette.
Sheldon becomes annoyed after discovering that Amy has been helping Kripke with his research. Stuart finally reopens the comic book store. Howard gets a disturbing phone call regarding his mother.
Penny agrees to be part of Sheldon's intimacy experiment. Meanwhile, the airline loses the ashes of Mrs. Wolowitz that Howard was bringing home from Florida.
Amy and Sheldon buy a pet turtle together. Sheldon plans on becoming a colonist on Mars someday. Penny and Leonard worry that they are becoming boring. Raj snoops around Emily's apartment while she's away for the evening.
When the loss of electricity in the Wolowitz house leads to the defrosting of the final meal cooked by Mrs. Wolowitz, the gang decides to consume it. Meanwhile a magazine publishes an article about Sheldon and Leonard's work but only Sheldon's name is mentioned in the article.
When Leonard and Sheldon are invited to speak at UC Berkeley, they take a detour to try to meet an idol. Also, when Bernadette and Howard battle over the fate of his Dr. Who TARDIS, Raj and Amy offer to settle the dispute with a game of ping pong.
Howard discovers a family secret. Sheldon and Amy build a fort. Penny receives an audition for a Kevin Smith movie.
The guys clash about a message that could be used for contacting aliens.
Leonard and Penny are headed to New Jersey, where Leonard is scheduled to give a high school commencement speech. Raj's father is tired of his reckless spending habits.
Sheldon's and Leonard's mothers finally come face to face and their personalities collide.
Sheldon wants Penny and Leonard to decide on a wedding date.