Leonard and Sheldon are brilliant physicists - geniuses in the laboratory but socially challenged everywhere else. Insert beautiful, street-smart neighbor Penny, who endeavors to teach them a thing or two about the 'real' world. Despite their on-again, off-again relationship history, Leonard and Penny have tied the knot. Even Sheldon has entered into a 'Relationship Agreement' with neurobiologist Amy, and he recently took their relationship to the next level by proposing marriage.