Sheldon's getting fired forces him to explore what life has to offer outside physics, leaving Leonard to take drastic action to snap his friend out of his funk.

With Wolowitz AWOL with one of her friends, Penny's recruited to fill in for him on the guys' Halo team

Raj's parents set him up on a blind date, but just when he discovers he can talk to her, she shows interest in Sheldon.

When they're invited to present at a physics conference, Sheldon's reluctance to share the credit leads to a tiff between him and Leonard, and Penny's intervention only exacerbates the situation.

Leonard and Sheldon each lie to avoid seeing Penny's concert, but Sheldon's is a bit too complicated for his own good.

The rest of the gang fall for Sheldon's twin sister, and Leonard's plan to win her heart is derailed when Sheldon declares he'll be the one to decide who can date her.

Penny's break-up spurs Leonard to summon the courage to ask her out, and each goes to Sheldon for advice on their upcoming date.

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

Leonard and his friends stage an intervention to force Sheldon to face his fear of driving.

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

After overhearing that Wil Wheaton is entering a trading-card competition, Sheldon decides to enter it himself. Meanwhile, Penny sets up one of her friends on a date with Wolowitz.

Leonard can only bring one guest on a trip to see CERN's Large Hadron Collider (the world's largest and highest energy particle accelerator) in Switzerland on Valentine's Day.

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

The guys are concerned as Sheldon searches for an alternative to human companionship.

An embarrassing secret of Wolowitz's comes to light, while Sheldon and Raj have a small war at work.

While the guys camp in line for a long-awaited movie screening, Penny and Bernadette invite Amy to her first slumber party.

Amy finds she has sexual feelings for Penny's ex, while Koothrappali and Wolowitz try to prove who would be the better superhero.

Bernadette runs into her ex-boyfriend while attending a science event with Wolowitz.