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On the sixth season premiere, Howard discovers that he can't escape his mother -- even in space! Back on earth, meanwhile, Raj turns his existence as a third wheel into an art form

When Sheldon learns that Penny is thinking about breaking up with Leonard, he tries to intervene. Meanwhile, Howard is being picked on in space by the other astronauts.

Amy's nervous when Sheldon hires Alex, a female assistant.

Howard returns from space...without the welcome he wanted.

The gang celebrates Halloween at Stuart's comic book store. Meanwhile Leonard and Penny discover a new spark in their relationship when he seduces her with science.

Sheldon's friendship with renowned scientist STEPHEN HAWKING is threatened by a game of "Words with Friends," while Penny secretly enrolls in a class at the local college.

Bernadette pressures Howard to move out of his mother's house, while Sheldon finds himself caught in the middle of a fight between Amy and Wil Wheaton.

Howard and Raj try to solve a mystery about Sheldon.

Sheldon & Howard battle over an assigned parking spot.

Howard bonds with his father-n-law on a fishing trip.

Sheldon unwillingly revisits some Christmas memories during a lively game of Dungeons and Dragons, while Raj joins the girls for ladies night out.

Sheldon is accused of sexual harassment at the University and winds up dragging Leonard, Raj and Howard into the fray.

Leonard's car is stolen when the guys take a road trip to a comic book convention in Bakersfield, dressed as Star Trek characters.

Outraged that he's being forced to work with his university nemesis, Barry Kripke (guest star JOHN ROSS BOWIE), Sheldon faces a crisis of conscience.

When Sheldon and Leonard fight, it affects Penny and Amy's living arrangements. Meanwhile, Raj takes care of Mrs. Wolowitz while Howard is away.

Sheldon struggles to find a Valentine's gift for Amy.

Raj vows never to leave his apartment after a horrible date.

Leonard, Sheldon and Howard try to advance the cause of women in science by speaking with junior high girls about their future careers, while Raj plans his date with Lucy.

To open or not to open? When Howard finds a letter from his father, he's forced to confront his past. And Leonard and Penny throw a "grown-up" cocktail party at the apartment.

Leonard, Sheldon and Raj fight for tenure at the University.

After one of his favorite TV shows gets canceled too soon, Sheldon feels lost, so Amy tries to help him get over his compulsive need for closure by retraining his brain.

Leonard and Sheldon hire Professor Proton (guest star BOB NEWHART), the host of their favorite childhood TV show to perform. Raj leaves his dog with Howard and Bernadette.

When the girls' trip to Vegas falls through, the guys invite them to a Dungeons & Dragons date night that causes Sheldon and Amy's relationship to take an unexpected turn.

On the sixth season finale, Sheldon and Penny are thrown for a loop when Leonard is offered an exciting new job opportunity overseas.