After more than nine years together, the gang finally celebrates Sheldon’s birthday, surprising him with a special guest.

While Penny worries that dating Leonard has ruined her for normal guys, Wolowitz and Koothrappali search for Sheldon’s perfect match online.

A paintball game leads to a fight between Sheldon and Penny, and romance for Leslie and Wolowitz.

Sheldon and the gang are confused when a young grad student is attracted to him.

Sheldon creates a monster when he introduces Penny to online gaming.

A jealous Leonard reacts to Penny’s new guy by rebounding with Leslie.

Sheldon finds himself booted from the gang’s Physics Bowl team and replaced with Leslie, so he forms his own team, hoping to prove he’s superior.

Fearing he’s wasting his time on Penny, Leonard ends up making a connection with Leslie.

Leonard asks a woman out after he finds out that Penny is seeing someone.