Leonard and Sheldon are brilliant physicists - geniuses in the laboratory, but socially challenged everywhere else. Enter their 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 and they have finally gotten engaged. 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.