This season develops the character of Amy Farrah Fowler, a character who was intended to be the female version of Sheldon. While her interactions with Sheldon throughout the season are priceless, the character really hits her stride by bonding with Penny, her new "bestie" (i.e. "best friend").
In addition, the season moves beyond the show's initial Leonard/Penny relationship formula by having Leonard get into a mostly-stable relationship with Raj's sister, Priya. As the season approaches its climax, Howard and Bernadette get engaged, setting the stage for a hilarious cliffhanger ending involving Raj and Penny.
- "The Robotic Manipulation"
- "The Cruciferous Vegetable Amplification"
- "The Zazzy Substitution"
- "The Hot Troll Deviation"
- "The Desperation Emanation"
- "The Irish Pub Formulation"
- "The Apology Insufficiency"
- "The 21-Second Excitation"
- "The Boyfriend Complexity"
- "The Alien Parasite Hypothesis"
- "The Justice League Recombination"
- "The Bus Pants Utilization"
- "The Love Car Displacement"
- "The Thespian Catalyst"
- "The Benefactor Factor"
- "The Cohabitation Formulation"
- "The Toast Derivation"
- "The Prestidigitation Approximation"
- "The Zarnecki Incursion"
- "The Herb Garden Germination"
- "The Agreement Dissection"
- "The Wildebeest Implementation" - Sheldon develops 3-person chess, Bernadette spies on Priya & Leonard's relationship, and Raj tries some anti-anxiety medication with unfortunate results.
- "The Engagement Reaction" - Howard tells his mother about his engagement to Bernadette, which lands her (his mother) in the hospital.
- "The Roommate Transmogrification" - Raj becomes Sheldon's new roommate, Leonard and Priya's relationship runs into a new bump, and Penny finders herself in an unexpected predicament.