Johnny Galecki and Kaley Cuoco will lead fans on a trip down memory lane, sharing some of the best-kept backstage secrets and personal memories from the past 12 years.

Penny starts dating Leonard's handsome colleague; Leonard and his friends are tortured by Sheldon's obsession with gift-giving etiquette.

When Amy's parents and Sheldon's family arrive, everybody is focused to make sure all wedding arrangements go according to plan – everyone except the bride and groom.

Sheldon and Amy await big news. Meanwhile, Penny and Leonard have some big news of their own. Also, Howard and Bernadette face the stressful challenge of leaving their kids for the first time and Raj makes a new friend, as the gang travels together into an uncharted future.

Leonard and Penny are drawn together after a disastrous visit from Leonard's mother.

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

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

Leonard and Koothrappali escort a heartbroken Wolowitz to Las Vegas, and Sheldon becomes locked out of his apartment and must stay with Penny.

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

With Leonard, Howard and Raj away camping in the desert, an injured Penny has only Sheldon to rely on.

Leonard tells Penny about how he met Sheldon for the first time...and what happened to the elevator.

After more than five years of dating, Sheldon and Amy spend their first night together. Also, Leonard, Wolowitz, and Koothrappali must decide who will take their extra Star Wars movie ticket.

Penny's first date with Leonard goes awry; Penny finds Sheldon to be an unwilling confidant.

The guys' plan to work over the summer at the North Pole makes Penny revaluate her feelings for Leonard.

When Howard dresses up as Sheldon for Halloween, Sheldon seeks retaliation at Leonard and Penny's Halloween party. Leonard is shocked that Penny doesn't remember their first kiss.

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

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

Penny's home business turns into a complete nightmare when Sheldon takes over.

Penny squares off with a new beautiful female neighbor who may become the building's "new Penny."

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.

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

Leonard inadvertently steals away the new woman Wolowitz thinks is the love of his life.

Sheldon is thrilled when Leonard starts dating a woman who meets Sheldon's stringent standards.

Sheldon's solution to Penny's financial difficulty leads to a confrontation between Leonard and Penny's ex-boyfriend.

Leonard, Sheldon and Wolowitz are ecstatic to learn that a famous science-fiction actress (Summer Glau) is on board the train they are taking to San Francisco.