The shy Professor Sherman Klump is getting married. And the Klump family couldn't be more delighted. His fiancee Denise is thrilled to become a Klump, and with her appetite she'll fit right in; Mama thinks it's fabulous; Papa's less grouchy; Granny's relieved that Sherman will finally be enjoying the pleasures of intimate relations; and Ernie has simmered down. But Buddy Love--well, Buddy that's another matter. Professor Klump's ferocious alter ego Buddy Love breaks loose just as Klump is getting ready to marry his colleague. Hoping to finally rid himself of the monstrous Buddy, Klump decides to utilize Denise's cutting-edge DNA research and extract Buddy's DNA from his system. But Buddy reacts by laying claim to the professor's astounding invention -- a revolutionary youth serum -- which Klump then hides in his family's home. But Buddy locates the "youth juice" and makes off with it, forcing Klump into a desperate race against time that may restore his family and win him back the woman he loves.