A Christmas visit from his mother has Leonard discovering she’s kept a lot of news from him that she shares with Sheldon and worrying if she’ll approve of his romance with Penny.

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.

After he’s humiliated on National Public Radio, Sheldon vows to destroy Kripke, while Wolowitz tries not to destroy his new relationship with Bernadette.

With Leonard, Howard and Raj away camping in the desert, an injured Penny has only Sheldon to rely on.

A squabble between Leonard and Penny sees Sheldon playing mediator, though out of his own incovenience from the sitiation rather than altruism.

Sheldon becomes Leonard’s teacher in order to understand football, while an incident with flying a kite threatens Howard and Raj’s friendship.

After overhearing that Wil Wheaton is entering a trading-card competition, Sheldon decides to enter it himself. Meanwhile, Penny sets up one of her friends on a date with Wolowitz.

Raj must find a new job or be sent back to India, but Sheldon has a solution, which leaves Leonard and Penny dealing with Howard as a third wheel.

Howard and Raj visit a Goth nightclub to pick up women while Sheldon attempts to build a better Penny using chocolate-based behavior modification.

Leonard and Penny struggle to recover from an awkward first hookup while Sheldon and Howard stake their best comic books on a bet to determine the species of a cricket.

When Sheldon learns the guys tampered with his expedition data he got from the arctic, he leaves to Texas in disgrace. This results in the guys following him, which threatens Leonard’s hope for some romantic time with Penny and the guys’ friendship with Sheldon.

The guys’ plan to work over the summer at the North Pole makes Penny revaluate her feelings for Leonard.

Penny’s second date with Stuart sees her making a comment that gives Leonard new hope for a relationship with her. Wolowitz realizes there’s a crucial error in a NASA project he’s working on.

Leonard and Koothrappali escort a heartbroken Wolowitz to Las Vegas, and Sheldon becomes locked out of his apartment and must stay with Penny.

A fellow comic book enthusiast’s hitting on Penny leads to Leonard and Howard going to a bar’s Ladies’ Night.

Penny squares off with a new beautiful female neighbor who may become the building’s “new Penny.”

Penny’s home business turns into a complete nightmare when Sheldon takes over.

Leonard, Sheldon and Wolowitz are ecstatic to learn that a famous science-fiction actress (Summer Glau) is on board the train they are taking to San Francisco.

A paintball game leads to a fight between Sheldon and Penny, and romance for Leslie and Wolowitz.

Leonard and Penny are drawn together after a disastrous visit from Leonard’s mother.

Sheldon’s solution to Penny’s financial difficulty leads to a confrontation between Leonard and Penny’s ex-boyfriend.

Sheldon creates a scientific procedure for making friends.

Penny’s criticism of Wolowitz’s romantic life pushes him into depression and AWOL for a fighting robot battle the rest of guys are depending on him for.

Penny starts dating Leonard’s handsome colleague; Leonard and his friends are tortured by Sheldon’s obsession with gift-giving etiquette.

Leonard asks Penny for advice when his new relationship starts moving too fast.

Sheldon is thrilled when Leonard starts dating a woman who meets Sheldon’s stringent standards.

Leonard inadvertently steals away the new woman Wolowitz thinks is the love of his life.

When Sheldon bans Penny from the apartment for numerous minor infractions, she decides to retaliate.

Sheldon and the gang are confused when a young grad student is attracted to him.

Leonard and his friends stage an intervention to force Sheldon to face his fear of driving.

Raj gets an inflated ego and makes a play for Penny after being referenced in a magazine article.

Sheldon creates a monster when he introduces Penny to online gaming.

A jealous Leonard reacts to Penny’s new guy by rebounding with Leslie.

Penny’s first date with Leonard goes awry; Penny finds Sheldon to be an unwilling confidant.

Penny’s break-up spurs Leonard to summon the courage to ask her out, and each goes to Sheldon for advice on their upcoming date.

Penny wants to throw a surprise party for Leonard, but Sheldon’s indulgence at an electronics store throws her off-track.

The rest of the gang fall for Sheldon’s twin sister, and Leonard’s plan to win her heart is derailed when Sheldon declares he’ll be the one to decide who can date her.

When delivery of another collectible leads to Penny’s lashing out at Leonard, he decides it’s time to give up his habit, much to his friends’ protestation.

Sheldon finds himself booted from the gang’s Physics Bowl team and replaced with Leslie, so he forms his own team, hoping to prove he’s superior.

With Sheldon rattled over meeting Dennis, a 15-year-old physicist who’s smarter than he, Leonard helps him look for a way to debunk Dennis’s research.