The Big Bang Theory, The Hofstadter Isotope

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 persistently 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 undergraduate student is attracted to him.

When Sheldon bans Penny from the apartment for numerous minor infractions, she decides to retaliate. Howard and Raj search for the America's Next Top Model house.

Sheldon wants to use an alternate version of rock-paper-scissors to help resolve conflicts. A girl Wolowitz has a major crush on falls for Leonard.

Sheldon is over-exuberant that Leonard finally has a girlfriend matching his high standards.

Leonard seeks Penny's counsel when his relationship with Stephanie takes off faster than he'd like. Sheldon's hypochondriac tendencies get the better of him.

Sheldon irks the gangs with his obsession with proper Christmas gift protocol. Leonard'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 the 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 loans money to Penny so she can pay her rent, 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 see Terminator: The Sarah Connor Chronicles' Summer Glau is also aboard their train to San Francisco for a scientific symposium. Sheldon frets over having forgotten a paper he wants to present to George Smoot.

Penny's business selling hair accessories 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 and angered when she senses the guys are being taken advantage of.

A fellow comic book enthusiast's hitting on Penny leads to Leonard, Raj 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.

Leonard is torn between being honest or self-serving when Stuart asks his advice for an upcoming second date with Penny. Wolowitz realises there's a crucial error in a toilet he designed for the International Space Station.

The guys receive a prestigious offer to work over the summer at the North Pole, and Penny's resultant behaviour leaves Leonard unsure if he should go.