VIDEO: Sneak Peek - Tonight's Episode of CBS's BIG BANG THEORY

On tonight's episode of CBS's THE BIG BANG THEORY titled 'The Proton Transmogrification,' Professor Proton helps Sheldon cope with grief, while Leonard turns a relationship milestone into a competition with Penny. Meanwhile, Amy and Bernadette make a "Star Wars Day" gift for the guys.

The episode airs tonight Thursday, May 1 (8:00-8:31 PM, ET/PT) on the CBS Television Network. Bob Newhart returns as Professor Proton.Get a sneak peek below!

Leonard and Sheldon are brilliant physicists – geniuses in the laboratory, but socially challenged everywhere else. Enter beautiful, street-smart neighbor, Penny, who aims to teach them a thing or two about life. Despite their on-again, off-again relationship in the past, Leonard believes he's found true love with Penny. Even Sheldon has found a female companion, entering into a "Relationship Agreement" with neurobiologist Amy Farrah Fowler. In their free time, Leonard and Sheldon continue to enjoy frequent trips to the comic book store and fantasy role-playing games with their ever-expanding universe of friends, including fellow scientists Koothrappali, Wolowitz and Wolowitz's adorable microbiologist wife, Bernadette.