Leonard and Sheldon are brilliant physicists, but none of that genius helps them interact with people, especially women. All this begins to change when a free-spirited beauty, Penny, moves in next door. Sheldon is content spending his nights playing Klingon Boggle with their socially dysfunctional friends. However, Leonard sees in Penny a whole new universe of possibilities ... including love.
After her first date with Leonard (JOHNNY GALECKI) goes awry, Penny (KALEY CUOCO) finds an unwilling confidante in Leonard's anti-social roommate, Sheldon (JIM PARSONS).
When Leonard sees Penny with a handsome new guy (TRAVIS SCHULDT), he enters into a rebound relationship with Sheldon's nemesis, Leslie Winkle (series regular SARA GILBERT).
Sheldon creates a monster when he introduces Penny to the world of online gaming. Especially when her addiction wreaks havoc on his life.
Newly famous because of an article in a magazine, Raj (series regular KUNAL NAYYAR) gets an inflated ego and makes a disastrous play for Leonard's longtime crush, Penny.
Leonard and his friends stage an intervention to make Sheldon confront his longstanding fear of driving.
A young grad student's attraction to Sheldon confuses everyone -- including Sheldon himself.
Sheldon and Penny engage in a battle of wills as Penny vows to get revenge on Sheldon when he bans her from the apartment for a series of petty infractions.
Wolowitz (series regular SIMON HELBERG) thinks he's found the love of his life (guest star SARA RUE) -- until she meets Leonard.
Sheldon can barely contain his joy when Leonard starts dating a woman (SARA RUE) who meets Sheldon's exacting standards.
Leonard just can't say no. When his relationship with new girlfriend, Stephanie starts moving too fast, Leonard turns to Penny for advice.
Leonard's handsome colleague starts dating Penny. And Leonard's friends 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 teammate 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 (guest star BRIAN PATRICK WADE).
A disastrous visit from Mrs. Hofstadter (special guest star CHRISTINE BARANSKI) brings Leonard and Penny closer together.
A paintball game has surprising consequences for Leonard and his friends when it leads to a fight between Penny and Sheldon - and romance for Wolowitz and Leslie.
A train trip to San Francisco takes a major detour when Leonard, Sheldon, Wolowitz and Raj discover that beautiful sci-fi actress SUMMER GLAU ("Terminator") is onboard.
Penny's home hair products business becomes a complete nightmare when a caffeine-addled Sheldon takes over.
There's a new queen bee in the building! Penny develops a rivalry with a sexy female neighbor moves in to the building and threatens to become the "new Penny."
While Stuart (guest star KEVIN SUSSMAN), a fellow comic book-loving pal of the guys, romances Penny, Leonard and Wolowitz venture out into the world of "Ladies' Night".
Leonard and Koothrappali take a heartbroken Wolowitz to Las Vegas, leaving Sheldon locked out of his apartment and forced to bunk with Penny.
Penny makes a surprising revelation about Leonard on her second date with Stuart. And, Wolowitz enlists his friends' help when he discovers a design flaw in his NASA project.
In the second season finale, Leonard and Penny reconsider their feelings for one another, when Leonard and his friends decide to spend the summer working - at the North Pole.