Penny opens up to a reluctant Sheldon after a less-than-stellar first date with Leonard, placing him in the uncomfortable position of having to keep a secret.
Seeing Penny out on a date with another guy sends a distressed Leonard rushing back to Leslie Winkle, much to Sheldon's dismay
Penny becomes addicted to an online game based on the world of Conan the Barbarian and persistently pesters Sheldon for advice.
Full of confidence after being named one of People Magazine's '30 to Watch under 30', Raj hits on Penny and becomes impossibly arrogant
When Leonard can't drive Sheldon to and from work, the friends stage an intervention to overcome Sheldon's fear of getting behind the wheel
Sheldon's friends are mystified when a younger student is attracted to him, but her intense devotion soon becomes too much
After Sheldon bans Penny from the apartment for numerous minor infractions, she decides to retaliate. Her plan involves a telephone, and Sheldon's mother.
A girl Wolowitz has a major crush on falls for Leonard, despite Wolowitz's elaborate attempts to woo her - including an abortive joyride on the Mars Rover.
Sheldon is delighted that Leonard finally has a girlfriend matching his high standards. Perhaps a little too delighted.
Leonard goes to Penny for help when his relationship with Stephanie takes off faster than he'd like, but will he choose to act on her advice?
At Christmas, Leonard's handsome colleague starts dating Penny, while Sheldon's obsession with gift-giving etiquette torments his friends
Kripke challenges the boys to a robot duel, while Howard is hurt when Penny reveals what she thinks about his behaviour towards women
Sheldon develops a scientific procedure for making friends when he discovers that Kripke has control of a new supercomputer at the university
When Sheldon lends Penny some money, it brings her ex-boyfriend Kurt back into her life, with negative consequences for the gang
A disastrous visit from Leonard's mother Beverly has the unexpected side-effect of bringing Leonard and Penny closer together
A game of paintball leads to a squabble when Penny accidentally shoots paint at one of Sheldon's cushions. Romance blossoms between Howard and Leslie.
Leonard, Sheldon, Howard and Raj discover that beautiful sci-fi actress Summer Glau is on board their train to San Francisco
Penny's home-based business selling hair barrettes - her 'Pennyblossoms' - gets out of hand when a caffeine-high Sheldon takes charge
Penny develops a rivalry with sexy new female neighbour Alicia, who threatens to steal the boys' attention and become the 'new Penny'
Leonard is devastated when Stuart from the comic book shop chats Penny up. Even worse, she doesn't mind, so Sheldon tries to scupper the budding romance.
Leonard and Raj take a heartbroken Howard to Las Vegas, home of slots, shrimp, and high-class hookers. Sheldon, meanwhile, is locked out of his apartment and forced to bunk with Penny.
Things begin to look up for Leonard when Penny makes a surprising revelation on her second date with Stuart
Leonard and Penny reconsider their feelings for each other, but then Leonard decides to spend the summer working at the North Pole - on a mission led by Sheldon.