After tampering with Sheldon's Arctic-expedition data, the guys follow him back to Texas, which threatens Leonard's chances for romance with Penny.
Penny and Leonard try to recover from an awkward hookup; Howard and Sheldon make a high-stakes bet.
Howard and Raj hit a Goth nightclub to meet women; Sheldon tries chocolate-based behavior modification on Penny.
Raj faces deportation if he does not find a job; Leonard and Penny deal with Howard as a third wheel.
Sheldon seeks revenge on his nemesis, Wil Wheaton; Wolowitz wants Penny to set him up with one of her friends.
Sheldon teaches Leonard how to understand football; a kite-fighting incident threatens Howard and Raj's friendship.
Sheldon tries to make peace when Leonard and Penny get into an argument.
Penny is forced to rely on Sheldon when she is injured while Howard, Leonard and Raj are away.
After being humiliated on the radio, Sheldon vows to destroy Kripke; Wolowitz tries to not ruin his new relationship with Bernadette.
Sheldon tries to help Penny understand Leonard's work; Wolowitz gets jealous when Leonard starts hanging out with his new girlfriend.
Leonard is horrified when his mother (Christine Baranski) visits.
Sheldon and Koothrappali attend a university mixer; Leonard discovers that Penny believes in psychics.
After their apartment is robbed, Leonard and Sheldon turn to their friends to create a state-of-the-art security system.
Sheldon tries to figure out the answer to a physics problem by working with Penny.
Leonard must choose who to bring with him when he goes to see the Large Hadron Collider on Valentine's Day.
Sheldon ends up in traffic court after getting a ticket because of Penny.
When the guys buy a ring from "The Lord of the Rings" at a garage sale, the item threatens to tear them apart.
Sheldon's stage fright stands between him and a coveted award.
Penny and Leonard have a fight that threatens their relationship; Sheldon and Wil Wheaton face off once again.
Sheldon goes to extremes to keep Leonard and Penny happy when they stop speaking to each other.
Sheldon invites a famous physicist to the apartment, and competes with Leonard for her attention.
Leonard tells Penny about how he met Sheldon and what happened to the elevator.