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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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.
S04, E16 (First Aired:
Wolowitz considers taking things with Bernadette to the next level; Leonard and Raj's sister rekindle their relationship.
S04, E17 (First Aired:
Sheldon is upset when he realizes Leonard is the center of their group of friends.
S04, E18 (First Aired:
Leonard has to choose between Penny and his new girlfriend, Priya; Howard's magic trick drives Sheldon crazy.
S06, E08 (First Aired:
Raj and Howard try to figure out why Sheldon disappears at 2:45 every afternoon; jealousy arises in Penny and Leonard's relationship.
S04, E20 (First Aired:
Amy and Sheldon spread gossip as part of an experiment; Howard decides to take a big step in his relationship with Bernadette.
S04, E22 (First Aired:
Raj tries to cure his social anxiety; Penny spies on Leonard and his girlfriend through Bernadette.
S04, E23 (First Aired:
Howard's mom winds up in the hospital after hearing news of her son's engagement to Bernadette.
S04, E24 (First Aired:
Bernadette gets her Ph.D., inspiring the guys to tease Wolowitz about his lack of a doctorate; Raj becomes Sheldon's roommate.
S05, E01 (First Aired:
Penny worries that she has ruined her relationship with her friends; Sheldon takes over the paintball team.
S05, E02 (First Aired:
Amy is caught in the middle when Penny and Sheldon get into an argument over a pre-owned chair; Leonard tries to make his long-distance relationship more exciting.
S05, E03 (First Aired:
Amy and Leonard try to become better acquainted; Bernadette, Howard and Mrs. Wolowitz spend the weekend together.
S05, E04 (First Aired:
Raj wants a girlfriend, so Penny introduces him to a woman he can communicate with.
S05, E05 (First Aired:
Wil Wheaton invites Leonard and Sheldon to a party at his house; Howard receives an opportunity.
S05, E06 (First Aired:
Sheldon finds himself competing with the gang for his mother's (Laurie Metcalf) attention when she visits.
S05, E07 (First Aired:
An attractive comic book artist tests Leonard and Priya's relationship; Sheldon tries to scare his friends.
Amy lies about repairs being made to her apartment so she can continue living with Sheldon; Leonard lets Penny decorate Sheldon's old room.
S05, E08 (First Aired:
Amy is devastated when Penny and Bernadette shop for wedding dresses without her.
S05, E10 (First Aired:
Sheldon is jealous when Stuart (Kevin Sussman), a guy from the comic book store, asks Amy out on a date.
S05, E11 (First Aired:
Leonard must face the bully who targeted him in high school; Penny confronts her past actions.
S05, E12 (First Aired:
Amy is upset with Sheldon when he isn't impressed with her recent accomplishment; Howard struggles with Bernadette's aversion to children.
S05, E13 (First Aired:
Everything could change when Leonard spontaneously invites Penny to a romantic dinner.
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.
S05, E14 (First Aired:
Penny and Leonard experiment with their new relationship; Raj forms a bond with his phone's virtual assistant.
S05, E15 (First Aired:
Leonard starts to reconsider his friendship with Sheldon; Wolowitz tries to choose his astronaut nickname.
S05, E16 (First Aired:
Sheldon goes to work with Amy in her lab when the university forces him to take a vacation; Bernadette asks Howard to sign a pre-nup.
S05, E17 (First Aired:
Sheldon and his nemesis, Kripke (John Ross Bowie), compete for a corner office; Penny feels uncomfortable after receiving a gift from Amy.
S05, E18 (First Aired:
Sheldon's routine is turned upside down when his barber gets sick; Wolowitz questions his decision to go into space.
S05, E19 (First Aired:
Sheldon plans a video game marathon with the guys instead of attending a birthday party for Amy's aunt.
S05, E20 (First Aired:
When Penny buys Leonard and Sheldon ``Star Trek'' collectibles, Sheldon finds himself haunted by Mr. Spock; after a blind date, Raj believes he has met his future wife.
S05, E21 (First Aired:
When Wolowitz works with Stephen Hawking, Sheldon is determined to meet the famous scientist.
S05, E22 (First Aired:
Knowledge of Howard's sexual history makes Bernadette reconsider their future together.
S05, E23 (First Aired:
Wedding plans are jeopardized when NASA reschedules Howard's mission; Leonard says something in the bedroom that surprises Penny.
S05, E24 (First Aired:
The gang tries to help Wolowitz and Bernadette get married before the space launch.
S06, E01 (First Aired:
Raj faces his loneliness when his friends are out on dates; while in space, Wolowitz gets drawn into a fight between Bernadette and his mom.
S06, E02 (First Aired:
Sheldon tries to intervene when he learns that Penny is thinking about breaking up with Leonard; Howard is picked on.