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When Sheldon gets news he's going to the North Pole on a NSF expedition, he invites the guys as his group.
Howard enlists the help of the boys in order to fix his space toilet. Stuart and Penny go on their second date.
When Wolowitz gets dumped by Leslie, Raj and Leonard take him to Las Vegas to get over his breakup. Meanwhile, Sheldon gets locked out of the apartment and has to crash at Penny's.
A friend of the guys who works at the comic book store, Stuart (Kevin Sussman) hits it off with Penny, prompting Leonard and Wolowitz to head out to ladies' night at a local bar.
Penny gets jealous of the guys' new sexy female neighbor (Valerie Azlynn)
The boys help Penny with her new business selling Penny Blossoms.
On a train ride to San Francisco, Leonard, Sheldon, Wolowitz, and Raj discover that Summer Glau is on their train ride.
A paintball game leads to a fight between Penny and Sheldon, but also for a romance between Wolowitz and Leslie!
Leonard's mother, Mrs. Hofstadter (Christine Branksi) comes into town and she ends up being exactly like Sheldon.
Sheldon lends Penny some money which makes Leonard go and try and collect some money from her ex-boyfriend Kurt for her.
In order to get time on the super computer at work that he needs for his experiment, Sheldon must befriend Kripke who is in charge of the device.
When Penny's comments about Wolowitz's love life sends him into depression, the boys are about to lose their most important teammate in an upcoming robot fighting competition.
It's Christmas time and Sheldon is annoying everyone with his theories on gift giving etiquette. Meanwhile, Leonard gets jealous when Penny stars dating one of his handsome co-workers.
When Sheldon and Penny convince Leonard that Stephanie has moved in, Sheldon tries to slow the relationship down.
Sheldon tries to secure Leonard's relationship with Stephanie and sets up a scenario where Leonard can prove he's the alpha male.
Wolowitz thinks he's found his soul mate, Dr. Stephanie Barnett (Sara Rue), until she meets Leonard. Wolowitz lets Stephanie drive the Mars rover, which gets stuck in a ditch.
When Penny's cable goes out, she starts going over the guys' apartment to watch America's Next Top Model. However, after violating some of Sheldon's arbitrary rules, he bands her from coming over.
After an open house night at the University with grad students, an obsessive student Ramona Nowitzki (Riki Lindhome) likes Sheldon and makes her way into becoming his girlfriend / assistant.
When Leonard needs to work the graveyard timeslot to gain access to the lasers for his experiment, Sheldon loses his ride to the university.
Raj is included in People's "30 under 30 to watch" list, prompting the rest of the group to be jealous.
When Penny locks herself out of the apartment, she comes to hang out with Sheldon in his as he plays Age of Conan. Penny, frustrated by her recent failures in life soon plays and becomes addicted to Conan.
The guys return from the Renaissance fair and see Penny has already moved on to a new guy, Eric (Travis Schuldt), prompting Leonard to pursue a relationship with prior fling Leslie Winkle.
Penny and Leonard return from their date from the end of season one and Leonard finds Howard and Raj spying on them via webcam.