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Bernadette gets her Ph.D., inspiring the guys to tease Wolowitz about his lack of a doctorate; Raj becomes Sheldon's roommate.

The Big Bang Theory

The Roommate Transmogrification

Howard's mom winds up in the hospital after hearing news of her son's engagement to Bernadette.

The Engagement Reaction

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Raj tries to cure his social anxiety; Penny spies on Leonard and his girlfriend through Bernadette.

The Wildebeest Implementation

Priya dissects the roommate agreement; the ladies take Sheldon dancing.

The Agreement Dissection

Amy and Sheldon spread gossip as part of an experiment; Howard decides to take a big step in his relationship with Bernadette.

The Herb Garden Germination

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The guys try to figure out who hacked Sheldon's online-game account.

The Zarnecki Incursion

Leonard has to choose between Penny and his new girlfriend, Priya; Howard's magic trick drives Sheldon crazy.

The Prestidigitation Approximation

Sheldon is upset when he realizes Leonard is the center of their group of friends.

The Toast Derivation

Wolowitz considers taking things with Bernadette to the next level; Leonard and Raj's sister rekindle their relationship.

The Cohabitation Formulation

A wealthy donor makes Leonard question what he is willing to do for science.

The Benefactor Factor

Sheldon takes acting lessons from Penny in hope of becoming a better teacher; Koothrappali fantasizes about his friend's girlfriend.

The Thespian Catalyst

Bernadette runs into her ex-boyfriend (Rick Fox) while attending a science event with Wolowitz.

The Love Car Displacement

Physicists and best friends Leonard and Sheldon learn about the social aspects of life through their relationships with the free-spirited beauty who moves in next door.

S4 E15 | The Benefactor Factor

E24 | The Roommate Transmogrification

E23 | The Engagement Reaction

E22 | The Wildebeest Implementation

E21 | The Agreement Dissection

E20 | The Herb Garden Germination

E19 | The Zarnecki Incursion

E18 | The Prestidigitation Approximation

E17 | The Toast Derivation

E16 | The Cohabitation Formulation

E15 | The Benefactor Factor

E14 | The Thespian Catalyst

E13 | The Love Car Displacement