After her first date with Leonard goes awry, Penny finds an unwilling confidant in Leonard's anti-social roommate, Sheldon.
When Leonard sees Penny with a handsome new guy, he enters into a rebound relationship with Sheldon's nemesis, Leslie Winkle.
Sheldon creates a monster when he introduces Penny to the world of online gaming.
Raj, newly famous because of an article in a magazine, gets a big head and makes a disastrous play for Leonard's crush, Penny.
Leonard and his friends stage an intervention to make Sheldon confront his longstanding fear of driving.
Young grad student's attraction to Sheldon confuses everyone, including Sheldon himself.
Penny vows to get revenge on Sheldon when he bans her from the apartment for a series of petty infractions.
Wolowitz thinks he's found the love of his life, until she meets Leonard.
Sheldon can barely contain his joy when Leonard starts dating a woman who meets Sheldon's exacting standards.
When Leonard's relationship with a new woman starts moving too fast, he goes to Penny for advice.
Christmas is a source of stress for Leonard - whose handsome colleague starts dating Penny - and his friends, who are being tormented by Sheldon's obsession with gift-giving etiquette.
When Penny's comment about Wolowitz's love life sends him into a depressed stupor, his friends lose their most important team-mate in an upcoming fighting robot competition.
Socially awkward Sheldon develops a scientific procedure for making friends.
Sheldon's "simple" solution to Penny's financial problem leads to a confrontation between Leonard and Penny's hulking ex-boyfriend, Kurt.
Disastrous visit from Mrs. Hofstadter brings Leonard and Penny closer together.
Paintball game leads to a fight between Penny and Sheldon, and romance for Wolowitz and Leslie.
Train trip to San Francisco takes a major detour when Leonard, Sheldon, Wolowitz and Raj discover that beautiful sci-fi actress Summer Glau is onboard.
Penny's home hair products business becomes a complete nightmare when a caffeine-addled Sheldon takes over.
Penny develops a rivalry with a sexy female neighbor who threatens to become "The New Penny" in the building.
While a fellow comic book-loving pal of the guys romances Penny, Leonard and Wolowitz venture out into the world of "Ladies' Night" at a local bar.
Leonard and Koothrappali take a heartbroken Wolowitz to Las Vegas, leaving Sheldon 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. Meanwhile, Wolowitz enlists his friends' help when he discovers a critical design flaw in his project for NASA.
Penny is conflicted when Leonard and his friends decide to spend the summer working - at the North Pole.