Leonard and Penny come to terms with their feelings for each other, as Leonard prepares himself for a three-month long science trip.

Things begin to look up for Leonard when Penny makes a surprising revelation on her second date with Stuart. Meanwhile, Wolowitz enlists his friends' help when he discovers a critical design flaw in his project for NASA.

Leonard and Koothrappali take a heartbroken Wolowitz to Las Vegas, leaving Sheldon locked out of his apartment and forced to bunk with Penny.

A fellow comic book enthusiast's hitting on Penny leads to Leonard and Howard going to a bar's Ladies' Night.

Penny feels threatened when another sexy woman moves into the apartment building.

Penny's business selling hair products out of her apartment gets out of hand when an over-caffeinated Sheldon takes charge.

The gang is thrilled when they learn Terminator: The Sarah Connor Chronicles' Summer Glau is also aboard their train to San Francisco.

Paintball leads to a squabble between Sheldon and Penny, as well as romance between Howard and Leslie.

Leonard suffers through a dreadful visit from his mother (Christine Baranski) that ends up bringing him and Penny closer.

Sheldon helps Penny with a financial crisis, but his solution leads to a confrontation between Leonard and Penny's burly ex-boyfriend Kurt (Brian Patrick Wade).

Sheldon creates a scientific procedure for making friends.

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.

Penny informs Sheldon that she intends to give gifts to him and Leonard on Christmas which happens to be around the corner. After considerable debate with Penny, Sheldon agrees to also give a gift to Penny on Christmas.

Leonard's relationship with Stephanie is moving too fast for him, so he runs to Penny for advice.

Sheldon is overexuberant that Leonard finally has a girlfriend matching his high standards.

A girl Wolowitz has a major crush on falls for Leonard.

When Sheldon bans Penny from the apartment for numerous minor infractions, she decides to retaliate.

Sheldon, and his friends, are mystified when a younger grad student is attracted to him.

An intervention is used to try to force Sheldon to overcome his fear of driving.

Full of confidence following getting a paper published, Raj hits on Penny.

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

Leonard begins dating Sheldon’s nemesis, Leslie Winkle, after seeing Penny with a handsome new guy.

Penny opens up to a reluctant Sheldon after a less-than-stellar first date with Leonard.