Amy og Leonard kjører til Arizona for å hente Sheldon, mens Penny drar på jobbintervju i firmaet til Bernadette. Howard blir ikke klok på det merkelige forholdet mellom Stuart og Mrs. Wolowitz.
Sheldon må undervise på universitetet, og gjengen blir overrasket når Howard melder seg på kurset hans. Amy utnytter den dårlige stemningen mellom Penny og Bernadette til sin fordel.
Howard kjenner på nervene når NASA ber ham om å kaste den første ballen under en baseballkamp. Leonard og Penny prøver å motbevise Sheldons påstand om at han og Amy er et bedre par enn dem.
Raj får kjenne på følgene av å være ærlig om sin egen fortid når kjæresten hans, Emily, gir Penny en kald skulder. Samtidig vurderer gutta å bli deleiere i tegneseriesjappa til Stuart.
NASA ber Raj om å lage en beskjed til utenomjordiske vesener, og gutta krangler om hva han bør gjøre. Penny vurderer å gjøre et comeback som skuespiller.
Leonard skal holde tale ved sin gamle high school, men må endre planene når flyet hans blir innstilt. Wolowitz og Sheldon sliter med å få en drone i lufta.
Sheldon og Leonards mødre møtes til slutt. Howard bestemmer seg for å gjøre sin del av husarbeidet, med litt hjelp fra vennene.
Sheldon presser Leonard og Penny til å velge en dato for bryllupet deres, samtidig som han må takle dramatiske endringer i forholdet til Amy.
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.
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 embarrassing 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 spot 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.