The guys' prestigious offer to work over the summer at the North Pole—and Penny's resultant behavior—leaves Leonard unsure if he should go.

Leonard is torn between being honest or self-serving when Stuart asks his advice for an upcoming second date with Penny. Wolowitz realizes there's a crucial error in a toilet he designed for the International Space Station.

Leonard and Raj take Wolowitz to Las Vegas to help him get over his break-up with Leslie. Sheldon, home alone and looking forward to it, locks himself out and has only one other place to stay—Penny's.

A fellow comic book enthusiast's hitting on Penny leads to Leonard, Raj, and Howard going to a bar's Ladies' Night.

Penny feels threatened when another sexy woman moves into the apartment building and angered when she senses the guys are being taken advantage of.

Penny's business selling hair accessories out of her apartment gets out of hand when an over-caffeinated Sheldon takes charge.

The gang is thrilled when they see Terminator: The Sarah Connor Chronicles' Summer Glau is also aboard their train to San Francisco for a scientific symposium. Meanwhile, Sheldon frets over having forgotten a paper he wants to present to George Smoot.

Paintball leads to a squabble between Sheldon and Penny, as well as romance between Howard and Leslie.

A visit from Leonard's mother ends up bringing him and Penny closer.

When Sheldon loans money to Penny so she can pay her rent, it leads to Leonard coming face-to-face with her ex, Kurt.