Leonard and Amy get to know each other better, while Howard and Bernadette spend the weekend with Mrs. Wolowitz.

A fight between Sheldon and Penny leaves Amy caught in the middle, while Leonard tries to spice up his long-distance relationship with Priya.

Penny is worried with how damaged her friendship with the guys will be in the aftermath of her sleeping with Raj while Sheldon becomes the leader of the paintball team.

When Bernadette receives her PhD, Wolowitz feels emasculated. Raj and Leonard swap places and Penny does something she’s regrets.

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

When Leonard spends too much time in the shower with Priya, Sheldon charges Leonard with two violations of the roommate agreement. However, before he can get too far, Priya uses…

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

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.

Sheldon’s online-game account is hacked by Todd Zarnecki, who then proceeds to steal all his items prompting the guys to go on a quest to retrieve them. Penny, Bernadette and…

Sheldon is baffled by Wolowitz’s magic trick. Meanwhile, Leonard realizes dating Priya may make it impossible to continue being friends with Penny.

Sheldon becomes concerned when everybody begins to hang out at Raj’s apartment and discovers Leonard is the center of the group.

As Wolowitz contemplates taking things with Bernadette to the next level, Leonard rekindles his relationship with Raj’s sister.

A wealthy donor to the university makes Leonard consider how far he’s willing to go for the sake of science.

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.

Leonard’s idea for a smartphone app derails his friendship with Sheldon.

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

Amy finds she has sexual feelings for Penny’s ex, while Koothrappali and Wolowitz try to prove who would be the better superhero.

Penny asks Leonard to lie to her father, while Howard, Raj and Bernadette pull an all-nighter at a telescope.

While the guys camp in line for a long-awaited movie screening, Penny and Bernadette invite Amy to her first slumber party.

Sheldon’s answers during an FBI interview put Wolowitz’s security clearance in jeopardy.

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.

An embarrassing secret of Wolowitz’s comes to light, while Sheldon and Raj have a small war at work.

The guys are concerned as Sheldon searches for an alternative to human companionship.

Realizing he will not live long enough to download his consciousness into a robot body, Sheldon tries to extend his life span.

Penny tags along on Sheldon’s first ever date; Wolowitz finds a new use for a robotic arm.

While Penny worries that dating Leonard has ruined her for normal guys, Wolowitz and Koothrappali search for Sheldon’s perfect match online.

Leonard tells Penny about how he met Sheldon for the first time…and what happened to the elevator.

When Leonard and Penny aren’t speaking, Sheldon goes to extremes to keep them both happy.

Leonard and Sheldon compete for the attention of a famous female physicist.

A fight between Leonard and Penny threatens their relationship, while Sheldon battles Wil Wheaton in bowling.

Sheldon’s friends come to his aid when his fear of public speaking stands between him and a coveted award.

When the guys find a ring from “The Lord of the Rings” at a garage sale, it threatens to tear them apart.

Sheldon winds up in traffic court when he gets a ticket because of Penny.

Leonard can only bring one guest on a trip to see CERN’s Large Hadron Collider (the world’s largest and highest energy particle accelerator) in Switzerland on Valentine’s Day.

Sheldon’s search for the answer to a physics problem leads him to work at the Cheesecake Factory with Penny.

When their apartment is robbed, Leonard and Sheldon turn to their friends to create a state-of-the-art security system.

Leonard is aghast to find out a secret of Penny’s: she believes in psychics. Meanwhile, Sheldon tries to assist Koothrappali in picking up a girl at a university mixer.

Penny gets help from Sheldon in order to understand what Leonard’s work is, while Wolowitz develops a jealousy when Leonard starts hanging out with his new girlfriend.