Leonard and Amy make an unexpected trip to Arizona to fetch Sheldon; 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.
Wolowitz feels nervous when he is asked 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.
Raj and Sheldon try to simulate a dark-matter research expedition; after a fight about money, Penny and Leonard turn to Bernadette and Wolowitz for advice.
The guys try to help when Penny is faced with an awkward dilemma at work; Bernadette and Amy discuss sexuality and female scientists.
Sheldon feels pressured to participate in romantic traditions when the gang re-creates a high-school prom on the roof.
Leonard has minor surgery on his nose, causing Sheldon to overreact; Bernadette and Howard work on their marriage.
Sheldon says goodbye to Fun With Flags; Howard, Leonard and Raj look through their dead professor's research; Bernadette learns her co-workers' true feelings.
Leonard and Wolowitz clash over a work emergency; Amy throws a Victorian Christmas party which Sheldon resents.
When the guys agree to do something the girls like, Sheldon and Leonard end up having a confrontation.
Sheldon asks his friends to challenge him in hope of achieving a scientific breakthrough; Wolowitz invents a game about Raj.
Sheldon and Leonard must defend themselves from an online bully's criticism after they co-publish a paper; Bernadette, Penny and Amy confront embarrassing moments during a girls night.
While the gang waits for the reopening of Stuart's comic book store Wolowitz gets some shocking news; Penny has trouble taking her own advice; Leonard and Raj think they spot Nathan Fillion at a restaurant.
Penny and Sheldon design an experiment -- as a joke -- to make participants fall in love; Howard and Bernadette get into trouble at the airport.
Amy finds out that Sheldon applied for a one-way mission to colonize Mars; a trip to the pet store leads to a fight; Leonard has a surprise for Penny; Raj is busted while snooping in Emily's apartment.
A magazine article only credits Sheldon in an article about a paper he co-wrote with Leonard; a blackout at Mrs. Wolowitz's results in a final dinner in her honor.
When Sheldon and Leonard are asked to speak at UC Berkeley, they take a detour in hope of meeting an idol; Raj and Amy try to settle a dispute between Howard and Bernadette with a game of pingpong.
Sheldon and Amy build a blanket fort -- and come to a crossroads in their relationship; Wolowitz meets a half-brother he didn't know he had; Leonard and Penny have an argument during an appearance on Wil Wheaton's podcast.
The guys fight over what message Raj should write for NASA in case of alien contact; Penny has to decide between auditioning for a movie or keeping her pharmaceutical sales job.
Wolowitz questions his engineering skills after he and Sheldon can't get a toy drone to fly; a canceled flight puts Leonard's plans in jeopardy; Raj sets his parents against one another after he's cut off financially.
When Sheldon's and Leonard's mothers finally meet, personalities collide; Howard decides to do his share of the chores -- with a little help from his friends.