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.
Sheldon is ordered to teach a class, and the gang is surprised when Howard signs up to take it; and tensions between Penny and Bernadette afford Amy an opportunity to play one against the other.
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".
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.
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.
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 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.
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.
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.
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.
On Christmas Eve, Amy plans a traditional Victorian dinner, while Raj, Leonard and Howard scramble to remove a pigeon that flew into the clean room at the university. To prove a point to Amy, a Scrooge-like Sheldon buys her the worst possible gift.
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?"
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.
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 Dr. 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.
In the Season 8 finale, Leonard and Penny are pressured to set a wedding date by Sheldon, whose relationship with Amy has taken an interesting turn.