Summary: Mensa-fied best friends and roommates Leonard and Sheldon, physicists who work at the California Institute of Technology, may be able to tell everybody more than they want to know about quantum physics, but getting through most basic social situations, especially ones involving women, totally baffles them. How lucky, then, that babe-alicious waitress/aspiring actress Penny moves in next door. Frequently seen hanging out with Leonard and Sheldon are friends and fellow Caltech scientists Wolowitz and Koothrappali. Will worlds collide? Does Einstein theorize in the woods?
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S02, E17 (First Aired:
Leonard, Sheldon and Wolowitz are ecstatic to learn that a famous science-fiction actress (Summer Glau) is aboard the train they are taking to San Francisco.
S02, E18 (First Aired:
Penny's home business turns into a complete nightmare when Sheldon takes over.
S02, E19 (First Aired:
Penny develops a rivalry with a sexy new female neighbor who threatens her position as queen bee of the building.
S02, E21 (First Aired:
Koothrappali and Leonard take Wolowitz to Las Vegas; Sheldon is forced to bunk with Penny when he gets locked out of his apartment.
S02, E22 (First Aired:
Penny makes a surprising revelation about Leonard while on her date with Stuart; Wolowitz finds a critical design flaw in his NASA project.
S02, E23 (First Aired:
While Leonard prepares for a three-month science expedition, he and Penny re-evaluate their feelings for each other.
S03, E01 (First Aired:
After tampering with Sheldon's Arctic-expedition data, the guys follow him back to Texas, which threatens Leonard's chances for romance with Penny.
S03, E02 (First Aired:
Penny and Leonard try to recover from an awkward hookup; Howard and Sheldon make a high-stakes bet.
S03, E03 (First Aired:
Howard and Raj hit a Goth nightclub to meet women; Sheldon tries chocolate-based behavior modification on Penny.
S03, E04 (First Aired:
Raj faces deportation if he does not find a job; Leonard and Penny deal with Howard as a third wheel.
S03, E05 (First Aired:
Sheldon seeks revenge on his nemesis, Wil Wheaton; Wolowitz wants Penny to set him up with one of her friends.
S03, E06 (First Aired:
Sheldon teaches Leonard how to understand football; a kite-fighting incident threatens Howard and Raj's friendship.
S03, E08 (First Aired:
Penny is forced to rely on Sheldon when she is injured while Howard, Leonard and Raj are away.
S03, E09 (First Aired:
After being humiliated on the radio, Sheldon vows to destroy Kripke; Wolowitz tries to not ruin his new relationship with Bernadette.
S03, E10 (First Aired:
Sheldon tries to help Penny understand Leonard's work; Wolowitz gets jealous when Leonard starts hanging out with his new girlfriend.
S03, E12 (First Aired:
Sheldon and Koothrappali attend a university mixer; Leonard discovers that Penny believes in psychics.
S03, E13 (First Aired:
After their apartment is robbed, Leonard and Sheldon turn to their friends to create a state-of-the-art security system.
S03, E14 (First Aired:
Sheldon tries to figure out the answer to a physics problem by working with Penny.
S03, E15 (First Aired:
Leonard must choose who to bring with him when he goes to see the Large Hadron Collider on Valentine's Day.
S03, E17 (First Aired:
When the guys buy a ring from ``The Lord of the Rings'' at a garage sale, the item threatens to tear them apart.
S03, E19 (First Aired:
Penny and Leonard have a fight that threatens their relationship; Sheldon and Wil Wheaton face off once again.
S03, E20 (First Aired:
Sheldon goes to extremes to keep Leonard and Penny happy when they stop speaking to each other.
S03, E21 (First Aired:
Sheldon invites a famous physicist to the apartment, and competes with Leonard for her attention.
S03, E22 (First Aired:
Leonard tells Penny about how he met Sheldon and what happened to the elevator.
S03, E23 (First Aired:
Raj and Howard set up an online dating profile for Sheldon and find the perfect woman.
S04, E01 (First Aired:
Penny tags along on Sheldon's first date; Wolowitz finds a new use for a robotic arm.
S04, E02 (First Aired:
Realizing he will not live long enough to download his consciousness into a robot body, Sheldon tries to extend his life span.
S04, E03 (First Aired:
When Sheldon searches for an alternative to human companionship, the guys become concerned.
S04, E04 (First Aired:
An embarrassing secret of Wolowitz's is revealed; Raj and Sheldon get into a war at work.
S04, E05 (First Aired:
Leonard realizes he is the only one in his group without a girlfriend; Sheldon wants to get rid of his girlfriend.
S04, E08 (First Aired:
While the guys camp out for a long-awaited movie screening, Penny and Bernadette invite Amy to her first slumber party.
S04, E09 (First Aired:
Penny asks Leonard to lie to her father (Keith Carradine); Howard, Raj and Bernadette pull an all-nighter at a telescope.
S04, E10 (First Aired:
Amy realizes she has feelings for Penny's ex; Koothrappali and Wolowitz try to prove who would be a better superhero.
S09, E21 (First Aired:
During a group get-together, a minor argument between Sheldon and Leonard turns into a heated debate, leaving the rest of the gang to choose sides.
S04, E12 (First Aired:
Leonard's idea for a smartphone application causes friction in his friendship with Sheldon.
S04, E13 (First Aired:
Bernadette runs into her ex-boyfriend (Rick Fox) while attending a science event with Wolowitz.
S04, E14 (First Aired:
Sheldon takes acting lessons from Penny in hope of becoming a better teacher; Koothrappali fantasizes about his friend's girlfriend.
S02, E11 (First Aired:
Penny starts dating Leonard's handsome colleague; Leonard and his friends are tortured by Sheldon's obsession with gift-giving etiquette.
S06, E11 (First Aired:
During a game, Sheldon revisits Christmas memories; Amy, Penny and Bernadette look for a date for Koothrappali.
S07, E11 (First Aired:
While Sheldon is in Texas, the rest of the gang realize how much he has changed their lives as they decorate the apartment Christmas tree.