Penny opens up to a reluctant Sheldon after a less-than-stellar first date with Leonard, placing him in the uncomfortable position of having to keep a secret.
Seeing Penny with another guy sends Leonard back to Leslie Winkle, much to Sheldon's dismay.
Penny becomes addicted to an online game based on the world of Conan the Barbarian and persistantly pesters Sheldon for advice.
Full of confidence following being named one of People's "30 to Watch Under 30", Raj hits on Penny and becomes impossibly arrogant.
An intervention is used to try to force Sheldon to overcome his fear of driving.
Sheldon, and his friends, are mystified when a younger grad student is attracted to him.
When Sheldon bans her from the apartment for numerous minor infractions, Penny decides to retaliate.
A girl Wolowitz has a major crush on falls for Leonard. Sheldon tries to improve rock-paper-scissors.
Sheldon is overexuberant that Leonard finally has a girlfriend matching his high standards.
When Leonard's relationship with a new woman starts moving too fast, he goes to Penny for advice.
Sheldon irks the gangs with his obession with proper Christmas gift protocol. Lenoard's crushed when Penny dates a colleague of his.
Penny's criticism of Wolowitz's romantic life pushes him into depression and AWOL for a fighting robot battle the rest of guys are depending on him for.
Looking to get friendly with Barry Kripke to gain access to a piece of high-tech lab equipment, Sheldon develops a flowchart process to make friends.
When Sheldon comes up with a plan to fix Penny's finances, it leads to Leonard coming face-to-face with her ex, Kurt.
A visit from Leonard's mother ends up bringing him and Penny closer.
Paintball leads to a squabble between Sheldon and Penny, as well as romance between Howard and Leslie.
The gang is thrilled when they learn Terminator: The Sarah Connor Chronicles' Summer Glau is also aboard their train to San Francisco.
Penny's business selling hair products out of her apartment gets out of hand when an over-caffeinated Sheldon takes charge.
Penny feels threatened when another sexy woman moves into the apartment building.
A fellow comic book enthusiast's hitting on Penny leads to Leonard and Howard going to a bar's Ladies' Night.
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.
Penny's second date with Stuart sees her making a comment that gives Leonard new hope for a relationship with her. Wolowitz realizes there's a crucial error in a NASA project he's working on.
The guys' plan to work over the summer at the North Pole makes Penny reevaluate her feelings for Leonard.