Episode 1 / The Robotic Manipulation

On the fourth season premiere, Penny (series star KALEY CUOCO) finds herself along for the ride on Sheldon's (series star JIM PARSONS) first date ever. Meanwhile, Wolowitz (series star SIMON HELBERG) finds a new use for a robotic arm. MAYIM BIALIK guest stars as Sheldon's date, Amy Fowler.

Episode 2 / The Cruciferous Vegetable Amplification

When Sheldon realizes he won't live long enough to download his consciousness into a robot body, he attempts to extend his lifespan. Apple Co-Founder STEVE WOZNIAK guest stars as Himself.

Episode 3 / The Zazzy Substitution

The guys are concerned as Sheldon searches for an alternative to human companionship after he and Amy (MAYIM BIALIK) break up. LAURIE METCALF reprises her role as Sheldon's mother.

Episode 4 / The Hot Troll Deviation

An embarrassing secret of Wolowitz's comes to light, while Sheldon and Raj have a small war at work. Battlestar Galactica's KATEE SACKHOFF and Star Trek's GEORGE TAKEI guest star as themselves as Wolowitz's love gurus. MELISSA RAUCH reprises her role as Howard's girlfriend, Bernadette.

Episode 5 / The Desperation Emanation

Leonard realizes he's the only one without a girlfriend, while Sheldon wants to get rid of his.

Episode 6 / The Irish Pub Formulation

A visit from Raj's sister (guest star AARTI MANKAD) creates tension among the guys -- especially when Leonard keeps an affair secret from Sheldon, Howard and Raj.

Episode 7 / The Apology Insufficiency

When an FBI agent (guest star ELIZA DUSHKU) interviews the guys for Wolowitz's security clearance, Sheldon's answers during the screening put Wolowitz's clearance in jeopardy.

Episode 8 / The 21-Second Excitation

When the guys camp in line for a screening, their plans are thwarted by Sheldon's greatest nemesis, WIL WHEATON (guest starring as himself). Meanwhile, Penny and Bernadette invite Amy to her first slumber party.

Episode 9 / The Boyfriend Complexity

Penny asks Leonard to lie to her father, Wyatt (guest star KEITH CARRADINE), while Howard, Raj and Bernadette pull an all-nighter at a telescope.

Episode 10 / The Alien Parasite Hypothesis

Amy finds she has sexual feelings for Penny's ex, Zack (guest star BRIAN THOMAS SMITH) while Koothrappali and Wolowitz try to prove who would be the better superhero.

Episode 11 / The Justice League Recombination

The guys find a "super" use for Penny's new boyfriend, Zack, when they enter a costume contest as the Justice League.

Episode 12 / The Bus Pants Utilization

Leonard's idea for a smartphone app derails his friendship with Sheldon as they fight for control of the project.

Episode 13 / The Love Car Displacement

Tension runs high when everyone is staying at the same hotel for a science event and Bernadette runs into her ex-boyfriend (guest star RICK FOX, three-time NBA champion with the Los Angeles Lakers).

Episode 14 / The Thespian Catalyst

Sheldon wants Penny to give him acting lessons, hoping that they will help him become a better teacher. Meanwhile, Koothrappali fantasizes about his best friend's girlfriend.

Episode 15 / The Benefactor Factor

A wealthy donor to the university makes Leonard consider how far he's willing to go for the sake of science. JESSICA WALTER (Arrested Development) guest stars as the widowed benefactress and JOSHUA MALINA (The West Wing) guest stars as the University President.

Episode 16 / The Cohabitation Formulation

As Wolowitz contemplates taking things with Bernadette to the next level, he must choose between his girlfriend and his mother. Meanwhile, Leonard rekindles his relationship with Raj's sister, Priya.

Episode 17 / The Toast Derivation

When the gang starts hanging out at Raj's and Sheldon realizes it's actually Leonard who is the center of their social group, Sheldon decides to get a new group of friends.

Episode 18 / The Prestidigitation Approximation

Leonard must choose between his new girlfriend, Priya, and his friendship with Penny. Meanwhile, Howard drives Sheldon crazy with a magic trick.

Episode 19 / The Zarnecki Incursion

When someone hacks Sheldon's online gaming account, the guys go on a quest to find the culprit.

Episode 20 / The Herb Garden Germination

Sheldon and Amy experiment on their friends by spreading gossip, while Howard takes a big step in his relationship with Bernadette.

Episode 21 / The Agreement Dissection

Priya uses her lawyer skills to pick apart the roommate agreement...and put Sheldon in his place. Meanwhile, the girls take Sheldon dancing.

Episode 22 / The Wildebeest Implementation

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

Episode 23 / The Engagement Reaction

When Howard finally breaks the news to his mom that he's engaged to Bernadette, it lands her in the hospital.

Episode 24 / The Roommate Transmorgrification

When Bernadette receives her Ph.D., the guys take joy in reminding an emasculated Wolowitz that he is the only one in their group who isn't a doctor. Meanwhile, Koothrappali becomes Sheldon's new roommate after hearing Leonard and Priya engaging in a Star Trek bedroom fantasy, on the fourth season finale.