Leonard and Amy make an unexpected trip to Arizona to bring Sheldon home; Penny interviews for a job at Bernadette's company; Stuart's relationship with Mrs. Wolowitz disturbs Howard.

When Sheldon must teach a class, Howard decides to take it; Amy takes advantage of the tension between Penny and Bernadette by playing both sides.

Wolowitz feels nervous when he is asked by NASA to throw the first pitch at a Los Angeles Angels game; Penny and Leonard want to prove Sheldon wrong after he says he and Amy are the better couple.

Raj's honesty comes back to haunt him when his new girlfriend is unfriendly to Penny; the guys think about becoming part owners in Stuart's store.

During their weekend in Las Vegas, Bernadette and Amy accuse Penny of being a buzz kill; the guys try to invent the next big thing, but only succeed in procrastination.

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 deep underground. Leonard and Penny seek advice from Howard and Bernadette after they fight about money.

The guys try to help Penny out of an awkward work dilemma, when a smitten doctor shows up at her door after her flirtatious sales techniques work a little too well. Meanwhile, Amy and Bernadette argue over whether or not female scientists should play up their sexuality.

When the gang recreates a high school prom night on the roof of the guys' apartment building, Sheldon feels pressure to participate in all the typical romantic traditions.

When Leonard gets minor surgery on his nose, Sheldon is the one in need of sympathy. Meanwhile, Raj's parents' impending divorce causes Howard and Bernadette to work on their own marriage.

While Sheldon says a tearful goodbye to the final installment of "Fun with Flags," Leonard, Howard and Raj search for something significant in a dead professor's research and Bernadette is shocked to learn of her co-workers’ true feelings from Penny.

Leonard and Howard turn to Raj for help in dealing with a work emergency when they nearly come to blows after blaming each other for a potential disaster at the university. Amy throws an authentic Victorian Christmas party, while Sheldon seeks revenge on her for making him celebrate the holidays.

Wolowitz tries to distract Raj, who is anxiously awaiting data from a space probe he helped launch. Meanwhile, Sheldon and Leonard go dress shopping with Penny and Amy.

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?"

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.

Wolowitz receives some shocking news as everyone prepares for the reopening of Stuart’s comic book store. Penny teaches Sheldon how to “let it go,” but can’t follow her own advice after she learns something infuriating about Amy. Also, Leonard and Raj think they've spotted Nathan Fillion at a restaurant.

As a joke, Sheldon and Penny try an experiment designed to make participants fall in love. Also, Leonard, Amy, Raj and Emily spend the evening trying to escape a room with a “zombie.” And, after returning from Mrs. Wolowitz’ funeral, Bernadette and Howard run into trouble at the airport.

Trouble arises when Amy learns Sheldon has applied for a one-way mission to colonize Mars. Also, a trip to the pet store ends in a fight for Sheldon and Amy, Leonard surprises Penny with a very “adult” purchase, and Raj is caught snooping in Emily’s apartment.

Friction ensues when a magazine fails to mention Leonard in an article about the paper he co-wrote with Sheldon. Also, 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 to meet an idol. Also, when Bernadette and Howard battle over the fate of his Doctor Who TARDIS, Raj and Amy offer to settle the dispute with a game of ping pong.

When Sheldon and Amy build a blanket fort, they come to major crossroads in their relationship. Also, Wolowitz meets the half-brother he never knew he had, and when Penny is invited to appear on Wil Wheaton’s podcast, she and Leonard have a huge, on-air argument.

When Raj is asked to create a message in case a NASA mission discovers alien life, the guys fight over what he should do. Also, Penny is torn between auditioning for a movie or keeping her successful pharmaceutical sales job.

Wolowitz questions his engineering abilities when he and Sheldon can’t get a toy drone to fly. Meanwhile, a cancelled flight nearly prevents Leonard from giving the commencement address at his former high school, and Raj pits his parents against each other when his father cuts him off financially.

Personalities collide when Sheldon and Leonard’s mothers finally meet. Meanwhile, Howard finally decides to “man up” and do his fair share of the housework – but not without a little help from his friends.

Sheldon pushes Leonard and Penny to choose a date for their wedding and deals with dramatic changes in his own relationship with Amy.