Season 2 - Episode 1 / "THE BAD FISH PARADIGM"

After her first date with Leonard (series star JOHNNY GALECKI) goes awry, Penny (series star KALEY CUOCO) finds an unwilling confidante in Leonard's anti-social roommate, Sheldon (series star JIM PARSONS). An embarrassing secret threatens to derail their budding romance on the second season premiere.

Episode 2 / The Codpiece Topology

Penny and Leonard compete to see who will be the first to move on after their breakup. When Leonard sees Penny with a handsome new guy (guest star TRAVIS SCHULDT - "Scrubs"), he enters into a rebound relationship with Sheldon's nemesis, Leslie Winkle (series regular SARA GILBERT).

Episode 3 / The Barbarian Sublimation

Sheldon creates a monster when he introduces Penny to the world of online gaming. Especially when her addiction wreaks havoc on his life.

Episode 4 / The Griffin Equivalency

Raj's ego puts all of his friendships in jeopardy when he gets an inflated sense of self. Newly famous because of an article in a magazine, Raj (series regular KUNAL NAYYAR) makes a disastrous play for Leonard's longtime crush, Penny.

Episode 5 / The Euclid Alternative

Leonard and his friends stage an intervention to make Sheldon confront his longstanding fear of driving.

Episode 6 / The Cooper-Nowitzki Theorem

A young grad student's attraction to Sheldon confuses everyone -- including Sheldon himself.

Episode 7 / The Panty Pi�ata Polarization

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.

Episode 8 / The Lizard-Spock Expansion

Wolowitz (series regular SIMON HELBERG) thinks he's found the love of his life (guest star SARA RUE) -- until she meets Leonard.

Episode 9 / The White Asparagus Triangulation

Sheldon can barely contain his joy when Leonard starts dating a woman (SARA RUE) who meets Sheldon's exacting standards.

Episode 10 / The Vartabedian Conundrum

Leonard just can't say no. When his relationship with new girlfriend, Stephanie, starts moving too fast, Leonard turns to Penny for advice.

Episode 11 / The Bath Item Gift Hypothesis

Christmas is a source of stress for Leonard -- whose handsome colleague (MICHAEL TRUCCO) starts dating Penny -- and his friends, who are being tormented by Sheldon's obsession with gift-giving etiquette.

Episode 12 / The Killer Robot Instability

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.

Episode 13/ The Friendship Algorithm

Socially awkward Sheldon develops a scientific procedure for making friends.

Episode 14 / The Financial Permeability

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).

Episode 15 / The Maternal Capacitance

A disastrous visit from Mrs. Hofstadter (special guest star CHRISTINE BARANSKI) brings Leonard and Penny closer together.

Episode 16 / The Cushion Saturation

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.

Episode 17 / The Terminator Decoupling

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: The Sarah Connor Chronicles") is onboard. Nobel Laureate for Physics and "Big Bang" theorist, Dr. George Smoot also makes a cameo.

Episode 18 / The Work Song Nanocluster

Penny's home hair products business becomes a complete nightmare when a caffeine-addled Sheldon takes over.

Episode 19 / The Dead Hooker Juxtaposition

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."

Episode 20 / The Hofstadter Isotope

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" at a local bar.

Episode 21 / The Vegas Renormalization

Leonard and Koothrappali take a heartbroken Wolowitz to Las Vegas, leaving Sheldon locked out of his apartment and forced to bunk with Penny.

Episode 22 / The Monopolar Expedition

On 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 - Penny is conflicted.

Episode 23 / The Classified Material Turbulence

Things begin to look up for Leonard when Penny makes a surprising revelation on her second date with Stuart (guest star KEVIN SUSSMAN). Meanwhile, Wolowitz enlists his friends' help when he discovers a critical design flaw in his project for NASA.