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.
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.
Unable to get Comic-Con tickets, Sheldon tries to hold his own convention and ends up spending a wild evening with James Earl Jones; the ladies try to act maturely.
Sheldon and Amy go on a trip to wine country with Howard and Bernadette; Penny and Leonard rush Raj's dog to the vet.
When Leonard buys a dining room table, Sheldon re-evaluates the changes in his life; Bernadette is torn when Wolowitz gets the chance to go back to space.
Bernadette tries to negotiate peace between Sheldon and Wolowitz; Penny receives an embarrassing offer; Raj wants Amy to write to a woman on his behalf.
Sheldon is faced with a new reality when he visits his mom in Houston; a murder mystery party starts arguments.
Raj struggles with guilt when he dates two women at once; Penny has a chance to play a part in a cheesy movie; Sheldon is torn between two gaming systems.
Sheldon has a personal crisis when he decides string theory is a waste of his time; a double date ends in embarrassment for Howard.
Sheldon gives spontaneity a try, which causes unexpected friction for Penny, Bernadette and Amy; Raj asks Howard for help with preparation for a date with Emily.
Professor Proton (Bob Newhart) helps Sheldon deal with grief; Leonard gets into a competition with Penny; Bernadette and Amy prepare a "Star Wars Day" gift for the guys.
A bad day compels Penny to re-evaluate her choices -- including Leonard; Bernadette and Howard must care for Mrs. Wolowitz; Sheldon gives Raj advice on relationships.
Faced with an uncertain future, Sheldon considers a major change; Emily and Raj decide to take things to the next level.
Penny tags along on Sheldon's first date; Wolowitz finds a new use for a robotic arm.
Realizing he will not live long enough to download his consciousness into a robot body, Sheldon tries to extend his life span.
When Sheldon searches for an alternative to human companionship, the guys become concerned.
An embarrassing secret of Wolowitz's is revealed; Raj and Sheldon get into a war at work.
Leonard realizes he is the only one in his group without a girlfriend; Sheldon wants to get rid of his girlfriend.
Leonard keeps an affair a secret from Sheldon, Howard and Raj.
Sheldon's interview with the FBI jeopardizes Wolowitz's security clearance.
While the guys camp out for a long-awaited movie screening, Penny and Bernadette invite Amy to her first slumber party.
Penny asks Leonard to lie to her father (Keith Carradine); Howard, Raj and Bernadette pull an all-nighter at a telescope.
Amy realizes she has feelings for Penny's ex; Koothrappali and Wolowitz try to prove who would be a better superhero.
The guys try to bond with Penny's new boyfriend through a costume contest.
Leonard's idea for a smartphone application causes friction in his friendship with Sheldon.
Bernadette runs into her ex-boyfriend (Rick Fox) while attending a science event with Wolowitz.
When the guys buy a ring from "The Lord of the Rings" at a garage sale, the item threatens to tear them apart.
Sheldon's stage fright stands between him and a coveted award.
Penny and Leonard have a fight that threatens their relationship; Sheldon and Wil Wheaton face off once again.
Sheldon goes to extremes to keep Leonard and Penny happy when they stop speaking to each other.
Sheldon invites a famous physicist to the apartment, and competes with Leonard for her attention.
Leonard tells Penny about how he met Sheldon and what happened to the elevator.