Leonard must choose between Sheldon and Penny when he can bring only one guest on a trip to see the world's largest and highest energy particle accelerator in Switzerland.
Suffering from a case of "physicist's block," Sheldon's search for the answer to a physics problem leads him to work at the Cheesecake Factory with Penny.
When their apartment is robbed, Leonard and Sheldon turn to their friends to create a state-of-the-art security system - and Sheldon struggles to cope.
While Sheldon and Koothrappali attend a university mixer, Leonard is upset to discover that Penny believes in psychics. DANICA McKELLAR stars as the object of Raj's attraction.
A visit from Leonard's mother (guest star CHRISTINE BARANSKI) delights Sheldon and horrifies Leonard.
Sheldon takes on his greatest challenge when he attempts to help Penny understand physics, and Wolowitz becomes jealous when Leonard starts hanging out with his new girlfriend.
After he's humiliated on NPR, Sheldon vows to destroy Kripke, while, Wolowitz tries not to destroy his new relationship with Bernadette. KATEE SACKHOFF guest stars.
With Leonard, Howard and Raj away camping in the desert, an injured Penny has only Sheldon to rely on.
Sheldon plays the peacemaker when an argument between Leonard and Penny proves "inconvenient" to him.
Leonard learns about football from an unlikely tutor.
While Sheldon settles a score with his nemesis, Star Trek's WIL WHEATON (guest starring as himself), Wolowitz begs Leonard to get Penny to set him up with one of her friends.
Raj must find a new job or be sent back to India. Sheldon, however, has a solution - which leaves Leonard and Penny dealing with Howard as third wheel.
Howard and Raj (series star KUNAL NAYYAR) visit a Goth nightclub to pick up women, while Sheldon attempts to build a better Penny using chocolate-based behavior modification.
Leonard and Penny struggle to recover from an awkward first hookup, while Sheldon and Howard (SIMON HELBERG) stake their best comic books on a bet to determine an insect species.
Sheldon flees back to Texas in disgrace when he learns that the guys tampered with his arctic expedition data - threatening Leonard's hopes for a romantic encounter with Penny.
In the second season finale, Leonard and Penny reconsider their feelings for one another, when Leonard and his friends decide to spend the summer working - at the North Pole.