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Penny breaks up with her boyfriend after he posted intimate details about her on his blog. Leonard tries to comfort her but accidentally sends her back into the arms of her boyfriend.
When the boys go to eat at Penny's restaurant, she learns Leonard has never had a birthday before. She decides to throw him a surprise party and blackmails Leonard into helping.
The guys meet Sheldon's extremely attractive twin sister Missy (Courtney Henggeler) who is in town for a wedding.
Leonard buys a replica time machine online from the 1960 film The Time Machine and ends up splitting it with the guys.
The guys compete in a physics bowl after the previous contestants dropped out to form a barbershop quartet. When Sheldon's showboating becomes too much for them, they kick him off the team.
Sheldon becomes insecure over his intelligence because of a smarter 15-year-old prodigy Dennis Kim. He decides his work is useless and he aims for the Nobel Peace Prize by creating a "Nuevo Jerusalem."
Penny returns from a visiting family in Nebraska with a cold causing Sheldon to freak out because he's a bit of a germaphone.
When Penny is given a part in Rent, the boys try to avoid to see her show after hearing her sing "Out Tonight" out of key in order to not hurt her feelings.
After cleaning up after an internet experiment, Leonard discovers a letter in the trash can inviting the roommates to present some research they've done to a physics conference.
Raj is scared when his parents arrange a blind date for him with a girl from his childhood as he's still unable to women. Penny takes up bar tending to make some extra money.
Penny's slutty friend, Chrissy (Brooke D'Orsay) from Nebraska, is in town and is seduced by Wolowitz. The pair ends up taking over Penny's apartment, forcing her to sleep on Leonard and Sheldon's couch.
The guys are invited to Penny's Halloween party and decide to go only when the idea of costumes are mentioned. At the party, Wolowitz and Leonard attempt to socialize until Penny's ex, Kurt, shows up and humiliates Leonard.
While eating at the Cheesecake Factory where Penny works, the guys run into Leslie Winkle, the girl who previously rejected Leonard. Leslie asks Leonard to play in her string quartet as the cellist, but what she really wants is him.
Sheldon is fired from his job at Caltech when he insults his boss Dr. Eric Gablehauser. Unemployed, Sheldon quickly spirals into depressions which Leonard tries to end by bringing over Sheldon's mother.
When Leonard witnesses Penny kissing a guy in front of their apartment, he is hurt and feels "rejected." The guys try to convince him to ask a girl out at work and when he's rejected by Leslie Winkle, he is even more depressed.
When Sheldon and Leonard head over to Penny's apartment to drop off a package that arrived for her, Sheldon is bothered by how messy her apartment is.
In the pilot, life is about to change for two geniuses, Sheldon and Leonard when the beautiful Penny moves in across the hall. Leonard sees potential love interest in Penny and offers to retrieve her television from her ex-boyfriend.