Episode 1 / The Date Night Variable

On the sixth season premiere, Howard discovers that he can't escape his mother — even in space! Caught in an argument between Bernadette and his mom (recurring guest voice CAROL ANN SUSI), Howard deals with woman trouble from the International Space Station. Back on earth, meanwhile, Raj faces his loneliness, by turning his existence as a third wheel into an art form.

Episode 2 / The Decoupling Fluctuation

When Sheldon learns that Penny is thinking about breaking up with Leonard, he tries to intervene. But Sheldon is no good at keeping secrets. Meanwhile, Howard is being picked on in space by the other astronauts (guest stars PASHA LYCHNIKOFF and MIKE MASSIMINO).

Episode 3 / The Higgs Boson Observation

While the pressures of being in space starts to take its toll on Howard, Amy feels threatened when Sheldon hires a young female assistant, Alex (guest star MARGO HARSHMAN).

Episode 4 / The Re-Entry Minimization

When Howard returns from space, he doesn't get the hero's welcome he expected. Meanwhile, Game Night turns into a battle of the sexes.

Episode 5 / The Holographic Excitation

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. And while Howard can't stop talking about space, it takes iconic astronaut BUZZ ALDRIN (guest starring as himself) to get him grounded again.

Episode 6 / The Extract Obliteration

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

Episode 7 / The Habitation Configuration

Pressured by Bernadette, Howard comes to terms with moving out of his mother's house. Meanwhile, Sheldon, who's caught in the middle of a feud between his "girlfriend" Amy and good friend WIL WHEATON, goes to Penny for advice. Star Trek's LeVAR BURTON guest stars.

Episode 8 / The 43 Peculiarity

Howard and Raj try to get to the bottom of why Sheldon disappears every afternoon at 2:45. Meanwhile, Leonard and Penny wrestle with jealousy in their relationship.

Episode 9 / The Parking Spot Escalation

When the University reassigns Sheldon's (unused) parking spot to Howard, the ensuing turf battle affects the whole gang.

Episode 10 / The Fish Guts Displacement

Sheldon plays nursemaid to a sick Amy, while Howard attempts to bond with his father-in-law (guest star CASEY SANDER) on an upcoming fishing trip.

Episode 11 / The Santa Simulation

Sheldon unwillingly revisits some Christmas memories during a lively game of Dungeons and Dragons, while Penny, Bernadette and Amy try to find the perfect girl for Raj when he joins the girls for ladies night.

Episode 12 / The Egg Salad Equivalency

Sheldon is accused of sexual harassment at the University and winds up dragging Leonard, Raj and Howard into the fray. Southland's REGINA KING guest stars as the University's Director of Employee Relations.

Episode 13 / The Bakersfield Expedition

While the guys take a road trip to a comic book convention in Bakersfield dressed as characters from Star Trek: The Next Generation, Leonard's car is stolen. Meanwhile, the girls stay home and try to appreciate the world of comics.

Episode 14 / The Cooper/Kripke Inversion

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. Meanwhile, Howard and Raj spend a thousand dollars on action figures of themselves.

Episode 15 / The Spoiler Alert Segmentation

A fight between Leonard and Sheldon drives Leonard to finally move in with Penny — not realizing how the shift will affect Penny and Amy's living arrangements. Meanwhile, Raj takes care of Mrs. Wolowitz while Howard is away.

Episode 16 / The Tangible Affection Proof

While the guys try to give their significant others the perfect Valentine's Day, Raj and Stuart throw a lonely people party at the comic book store.

Episode 17 / The Monster Isolation

After a terrible date, Raj vows to never leave his apartment again. Meanwhile, Penny actually impresses Sheldon with her acting skills.

Episode 18 / The Contractual Obligation Implementation

Back to school! 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 (guest star KATE MICUCCI).

Episode 19 / The Closet Reconfiguration

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

Episode 20 / The Tenure Turbulence

Let the battle begin! Leonard, Sheldon and Raj fight for tenure at the University — and the competition heats up when the girls get involved!

Episode 21 / The Closure Alternative

Sheldon feels lost after one of his favorite TV shows gets canceled too soon, so Amy tries to help him get over his compulsive need for closure. Meanwhile, Raj discovers a secret about his new love interest, Lucy.

Episode 22 / The Proton Resurgence

Sheldon and Leonard hire Professor Proton (guest star BOB NEWHART), the host of their favorite childhood TV show, to perform. Also, Howard and Bernadette run into trouble babysitting Raj's dog.

Episode 23 / The Love Spell Potential

When the girls' trip to Vegas falls through, the guys invite them to play Dungeons & Dragons — causing Sheldon and Amy's relationship to take an unexpected turn. Meanwhile, Raj and Lucy go on a very awkward date.

Episode 24 / The Bon Voyage Reaction

When Leonard is offered an exciting new job opportunity overseas, Sheldon grows very jealous and Penny is thrown for a loop. Meanwhile, Raj pushes Lucy a little too far in their relationship on the sixth season finale.