The Locomotion Interruption Leonard and Amy take an unexpected road trip to Arizona to pick up Sheldon. Meanwhile, Penny interviews for a job at Bernadette's company, and Howard is weirded out by Stuart's relationship with Mrs. Wolowitz.
The Junior Professor Solution When Sheldon is forced to teach a class, Howard surprises everyone by taking it. Meanwhile, the tension between Penny and Bernadette gives Amy a chance to play both sides.
The First Pitch Insufficiency Wolowitz is 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 after he says that he and Amy are a superior couple.
The Hook-Up Reverberation Raj's honesty about his past comes back to haunt him after his girlfriend, Emily, is cold to Penny. Also, the guys consider becoming part-owners in Stuart's comic book store.
The Focus Attentuation Amy and Bernadette accuse Penny of being a buzzkill after she gets an email from work during their Vegas weekend. Meanwhile, the guys try to invent something cool, but only come up with new ways to procrastinate.
The Expedition Approximation 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 Wolowitz and Bernadette after they fight about money.
The Misinterpretation Agitation The guys try to help Penny out of an awkward work dilemma. Meanwhile, Amy and Bernadette argue over whether or not female scientists should play up their sexuality.
The Prom Equivalency When the gang recreates a high school prom on the roof the of the guys' apartment building, Sheldon feels pressure to participate in all the typical romantic traditions.
The Septum Deviation 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.
The Champagne Reflection Sheldon says a tearful goodbye to "Fun with Flags." Also, Leonard, Howard and Raj look for something significant in a dead professor's research, and Bernadette is shocked to learn her co-workers' true feelings from Penny.
The Clean Room Infiltration Leonard and Wolowitz nearly come to blows and turn to Raj for help in dealing with a work emergency. Meanwhile, Amy throws an authentic Victorian Christmas party, and Sheldon seeks revenge on her for making him celebrate the holidays.
The Space Probe Disintegration 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 Anxiety Optimization In the hopes of achieving a scientific breakthrough, Sheldon invites his friends to challenge him. Meanwhile, Wolowitz invents an embarrassing game about Raj.
The Troll Manifestation After Leonard and Sheldon co-publish a physics paper, they must defend themselves against criticism from an online bully. Also, girls' night finds Penny, Amy and Bernadette confronting embarrassing moments from their past.
The Comic Book Store Regeneration 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.
The Intimacy Acceleration 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.
The Colonization Application 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.
The Leftover Thermalization 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.
The Skywalker Incursion 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.
The Fortification Implementation 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.
The Communication Deterioration 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.
The Graduation Transmission 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.
The Maternal Combustion 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.
The Commitment Determination Sheldon pushes Leonard and Penny to choose a date for their wedding and deals with dramatic changes in his own relationship with Amy.