23 Episodes · Premiered September 21, 2009 · Ended May 24, 2010
Penny throws herself at Leonard after he returns from three months at the North Pole and they begin a relationship, Penny and Sheldon start a quirky friendship though she can still annoy him, Howard begins to date Bernadette Rostenkowski and Sheldon meets Amy Farrah Fowler.
Sheldon flees back to Texas in disgrace when he learns the guys tampered with his arctic expedition data, forcing the guys to follow him and threatening Leonard's hopes for a romantic encounter with Penny.
Leonard and Penny struggle to recover from an awkward first hookup while Sheldon and Howard stake their best comic books on a bet to determine the species of a cricket.
Howard and Raj visit a Goth nightclub to pick up women while Sheldon attempts to build a better Penny using chocolate-based behavior modification.
Raj must find a new job or be sent back to India, but Sheldon has a solution, which leaves Leonard and Penny dealing with Howard as third wheel.
While Sheldon settles a score with his nemesis, Wil Wheaton, Wolowitz begs Leonard to get Penny to set him up with one of her friends.
Sheldon teaches Leonard how to understand football while a kite-fighting incident threatens Howard and Raj's friendship.
An argument between Leonard and Penny proves "inconvenient" to Sheldon.
With Leonard, Howard and Raj away camping in the desert, an injured Penny has only Sheldon to rely on.
After he's humiliated on National Public Radio, Sheldon vows to destroy Kripke, while Wolowitz tries not to destroy his new relationship with Bernadette.
Sheldon attempts to help Penny understand Leonard's work, and Wolowitz becomes jealous when Leonard starts hanging out with his new girlfriend.
Visit from Leonard's mother delights Sheldon and horrifies Leonard.
While Sheldon and Koothrappali attend a university mixer, Leonard is upset to discover that Penny believes in psychics.
When their apartment is robbed, Leonard and Sheldon turn to their friends to create a state-of-the-art security system.
Sheldon's search for the answer to a physics problem leads him to work at the Cheesecake Factory with Penny.
Leonard can only bring one guest on a trip to see CERN's Large Hadron Collider in Switzerland on Valentine's Day.
Sheldon winds up in traffic court when he gets a ticket because of Penny.
When the guys find a ring from "The Lord of the Rings" at a garage sale, it threatens to tear them apart.
Sheldon's friends come to his aid when his fear of public speaking stands between him and a coveted award.
Fight between Leonard and Penny threatens their relationship, while Sheldon battles Wil Wheaton in bowling.
When Leonard and Penny aren't speaking, Sheldon goes to extremes to keep them both happy.
Leonard and Sheldon compete for the attention of a famous female physicist.
Leonard tells Penny about how he met Sheldon for the first time...and what happened to the elevator.
While Penny worries that dating Leonard has ruined her for normal guys, Wolowitz and Koothrappali search for Sheldon's perfect match online.
