When Bernadette receives her PhD, Wolowitz feels emasculated. Raj and Leonard swap places and Penny does something she’s regrets.

The guys’ plan to work over the summer at the North Pole makes Penny revaluate her feelings for Leonard.

Raj attempts to cure his social anxiety disorder while Penny uses Bernadette to spy on Leonard and his girlfriend.

Penny’s second date with Stuart sees her making a comment that gives Leonard new hope for a relationship with her. Wolowitz realizes there’s a crucial error in a NASA project…

Leonard and Koothrappali escort a heartbroken Wolowitz to Las Vegas, and Sheldon becomes locked out of his apartment and must stay with Penny.

When Leonard receives an exciting opportunity to work overseas, Penny and Sheldon are thrown for a loop. Raj and Lucy’s relationship comes to a crossroad.

A fellow comic book enthusiast’s hitting on Penny leads to Leonard and Howard going to a bar’s Ladies’ Night.

When Leonard spends too much time in the shower with Priya, Sheldon charges Leonard with two violations of the roommate agreement. However, before he can get too far, Priya uses…

Sheldon and Amy decide to start several rumors about the others as a test. Meanwhile, Howard takes his relationship with Bernadette to even higher level.

Leonard tells Penny about how he met Sheldon for the first time…and what happened to the elevator.

Leonard and Sheldon compete for the attention of a famous female physicist.

The fifth season ends with Howard and Bernadette deciding to get married before his NASA mission, causing the gang to scurry into overdrive in an effort to stage the wedding….

Penny squares off with a new beautiful female neighbor who may become the building’s “new Penny.”

Sheldon is baffled by Wolowitz’s magic trick. Meanwhile, Leonard realizes dating Priya may make it impossible to continue being friends with Penny.

When NASA moves up Howard’s mission, he must face his fears and Bernadette’s dad, thus putting the wedding plans in jeopardy. Meanwhile, Leonard says something surprising to Penny in the…

Penny’s break-up spurs Leonard to summon the courage to ask her out, and each goes to Sheldon for advice on their upcoming date.

Leonard and Sheldon hire the host (Bob Newhart) of their favorite childhood TV show to perform; Bernadette and Howard run into trouble while dog-sitting for Raj’s pet.

Sheldon get upset about his career, the destruction of the comic book store and Leonard and Penny’s future living arrangements. Howard and Bernadette can’t keep hold of a caretaker for…

A fight between Leonard and Penny threatens their relationship, while Sheldon battles Wil Wheaton in bowling.

Sheldon becomes concerned when everybody begins to hang out at Raj’s apartment and discovers Leonard is the center of the group.

Leonard, Sheldon and Wolowitz are ecstatic to learn that a famous science-fiction actress (Summer Glau) is on board the train they are taking to San Francisco.

Penny wants to throw a surprise party for Leonard, but Sheldon’s indulgence at an electronics store throws her off-track.

Amy tries to help Sheldon get over his compulsive need for closure when one of his favorite TV shows is canceled; Raj discovers a secret about Lucy.

A paintball game leads to a fight between Sheldon and Penny, and romance for Leslie and Wolowitz.

As the gang celebrates Star Wars Day, Sheldon has difficulty dealing with the recent death of Arthur “Professor Proton” Jeffries.

When Wolowitz gets to work with Stephen Hawking, Sheldon is willing to do anything to meet his hero.

Sheldon’s friends come to his aid when his fear of public speaking stands between him and a coveted award.

As Wolowitz contemplates taking things with Bernadette to the next level, Leonard rekindles his relationship with Raj’s sister.

The rest of the gang fall for Sheldon’s twin sister, and Leonard’s plan to win her heart is derailed when Sheldon declares he’ll be the one to decide who can…

The girls get involved when Leonard, Sheldon and Raj compete for tenure at the University.

Sheldon must choose between joining Amy at her aunt’s birthday party or playing videogames all weekend with the guys.

When the guys find a ring from “The Lord of the Rings” at a garage sale, it threatens to tear them apart.

A wealthy donor to the university makes Leonard consider how far he’s willing to go for the sake of science.

To prepare for a date with Emily, Howard acclimates Raj to horror movies. Penny and Sheldon are upset when they discover Amy and Bernadette lied to avoid hanging out with…

Leonard and Penny are drawn together after a disastrous visit from Leonard’s mother.

When delivery of another collectible leads to Penny’s lashing out at Leonard, he decides it’s time to give up his habit, much to his friends’ protestation.

As Sheldon presses Leonard and Penny to set a wedding date, he encounters major changes in his own relationship with Amy. Meanwhile, Raj contemplates breaking up with Emily, and Howard…

Penny buys Leonard and Sheldon “Star Trek” collectibles as a thank-you, leading Sheldon to be haunted by Mr. Spock. Meanwhile, Raj decides he has met the “Future Mrs. Koothrappali” after…

Leonard, Sheldon and Howard give lectures on science to junior high students. The girls ditch work to go to Disneyland. Raj devises a date for Lucy that fixes their social…

Sheldon faces a personal crisis after deciding he’s wasting his time with string theory. Meanwhile, a double date between Raj, Emily and the Wolowitzes ends in embarrassment for Howard.