This season the Big Bang gang’s romantic universe expands. On the rebound from Penny, Leonard falls into the arms of Raj’s sister Priya. Sheldon gets a girlfriend, or rather a friend who is a girl: Amy, a dour neurobiologist who declares herself besties with Penny. Howard and Bernadette heat up. And so do Raj and Bernadette (at least in Raj’s Bollywood daydream).
Penny tags along on Sheldon's first ever date; Wolowitz finds a new use for a robotic arm.
Realizing he will not live long enough to download his consciousness into a robot body, Sheldon tries to extend his life span.
The guys are concerned as Sheldon searches for an alternative to human companionship.
An embarrassing secret of Wolowitz's comes to light, while Sheldon and Raj have a small war at work.
Leonard realizes he’s the only one without a girlfriend, while Sheldon wants to get rid of his.
Leonard keeps an affair a secret from Sheldon, Howard and Raj.
Sheldon's answers during an FBI interview put Wolowitz's security clearance in jeopardy.
While the guys camp in line for a long-awaited movie screening, Penny and Bernadette invite Amy to her first slumber party.
Penny asks Leonard to lie to her father, while Howard, Raj and Bernadette pull an all-nighter at a telescope.
Amy finds she has sexual feelings for Penny's ex, while Koothrappali and Wolowitz try to prove who would be the better superhero.
The guys find a "super" use for Penny's new boyfriend, Zack, when they enter a costume contest as the Justice League.
Leonard's idea for a smartphone app derails his friendship with Sheldon.
Bernadette runs into her ex-boyfriend while attending a science event with Wolowitz.
Hoping it will make him a better teacher, Sheldon has Penny give him acting lessons. Raj has fantasies about a friend's girlfriend.
A wealthy donor to the university makes Leonard consider how far he's willing to go for the sake of science.
As Wolowitz contemplates taking things with Bernadette to the next level, Leonard rekindles his relationship with Raj's sister.
Sheldon becomes concerned when everybody begins to hang out at Raj's apartment and discovers Leonard is the center of the group.
Sheldon is baffled by Wolowitz's magic trick. Meanwhile, Leonard realizes dating Priya may make it impossible to continue being friends with Penny.
The guys go all out to help Sheldon find the person responsible for hacking his online gaming account, while the girls' hatred for Priya increases.
Sheldon and Amy decide to start several rumors about the others as a test. Meanwhile, Howard takes his relationship with Bernadette to even higher level.
When Priya finds a loophole in the roommate agreement to throw Sheldon out for the night, Penny, Amy and Bernadette take him to a nightclub.
Raj attempts to cure his social anxiety disorder while Penny uses Bernadette to spy on Leonard and his girlfriend.
When Howard finally breaks the news to his mom that he's engaged to Bernadette, it lands her in the hospital.
When Bernadette receives her PhD, Wolowitz feels emasculated. Raj and Leonard swap places and Penny does something she's regrets.