A woman who moves into an apartment across the hall from two brilliant but socially awkward physicists shows them how little they know about life outside of the laboratory.
Penny starts dating Leonard's handsome colleague; Leonard and his friends are tortured by Sheldon's obsession with gift-giving etiquette.
Leonard and Penny are drawn together after a disastrous visit from Leonard's mother.
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.
When Sheldon bans Penny from the apartment for numerous minor infractions, she decides to retaliate. Meanwhile, Howard and Rajesh try to find the house where all the models from "America's Top Model" are staying.
Sheldon creates a monster when he introduces Penny to online gaming. Meanwhile, Sheldon disagrees with Leslie Winkle at work.
Sheldon and the gang are confused when a young grad student is attracted to him.
Penny squares off with a new beautiful female neighbor who may become the building's "new Penny."
Sheldon is thrilled when Leonard starts dating a woman who meets Sheldon's stringent standards.
Penny's first date with Leonard goes awry; Penny finds Sheldon to be an unwilling confidant.
Leonard inadvertently steals away the new woman Wolowitz thinks is the love of his life.
Raj gets an inflated ego and makes a play for Penny after being referenced in a magazine article.
Leonard asks Penny for advice when his new relationship starts moving too fast.
Penny's home business turns into a complete nightmare when Sheldon takes over.
Leonard and his friends stage an intervention to force Sheldon to face his fear of driving.
A paintball game leads to a fight between Sheldon and Penny, and romance for Leslie and Wolowitz.
Sheldon's solution to Penny's financial difficulty leads to a confrontation between Leonard and Penny's ex-boyfriend.
When a fellow comic book enthusiast hits on Penny it leads to Leonard and Howard going to a bar's on Ladies' Night.
Sheldon creates a scientific procedure for making friends.
Leonard, Sheldon and Wolowitz are ecstatic to learn that a famous science-fiction actress is on board the train they are taking to San Francisco.
A jealous Leonard reacts to Penny's new guy by rebounding with Leslie Winkle.
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.
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