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.

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.

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.

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

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

Sheldon protests when the gang drags him to Mrs. Wolowitz’s house for Thanksgiving. Meanwhile, Penny and Leonard must deal with a mistake from her past.

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

Penny tags along on Sheldon's first ever date; Wolowitz finds a new use for a robotic arm.

When the entire gang competes in a scavenger hunt designed by Raj, it brings out the best - and worst - in everyone.

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

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

The guys find a "super" use for Penny's new boyfriend, Zack, when they enter a costume contest as the Justice League.

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.

When Bernadette receives her PhD, Wolowitz feels emasculated. Raj and Leonard swap places and Penny does something she's regrets.

A cute comic book artist puts Leonard and Priya’s relationship to the test, while Sheldon tries to scare the guys for Halloween.

When Wolowitz gets to work with Stephen Hawking, Sheldon is willing to do anything to meet his hero.

The fifth season ends with Howard and Bernadette deciding to get married before his NASA mission, causing the gang to scurry into overdrive in an effort to stage the wedding. NASA astronaut Mike Massimino has a cameo as himself.

The gang is concerned when Sheldon’s former admirer, Dr. Ramona Nowitzki, resurfaces while Amy is away at Princeton.

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

Penny gets help from Sheldon in order to understand what Leonard's work is, while Wolowitz develops a jealousy when Leonard starts hanging out with his new girlfriend.

Sheldon's search for the answer to a physics problem leads him to work at the Cheesecake Factory with Penny.

When the guys find a ring from "The Lord of the Rings" at a garage sale, it threatens to tear them apart.