Professor Sherman Klump is getting married. And the Klump family could not be more delighted. But Buddy Love…well, Buddy is another matter.

Lovable Sherman thought he had seen the last of his alter-ego, but Buddy is back. Having done his best to keep Buddy buried in his subconscious, the portly and painfully-shy professor is horrified to find him resurfacing in untimely outbursts, and threatening his marriage plans to colleague Denise Gaines (Janet Jackson). Utilizing Denise’s cutting-edge DNA research, Sherman decides to rid himself of his monstrous nemesis once and for all by extracting Buddy’s DNA from his system.

But Buddy will not go quietly, bursting full-bodied into Sherman’s world and laying claim to the professor’s astounding invention – a revolutionary youth serum. Desperate to keep the serum from Buddy, Sherman hides the serum in the Klump family home, thinking it will be safe. Buddy tracks the serum to the Klump home, where Denise’s bachelorette party is in full swing. Buddy quickly manages to locate the “youth juice” and steals away. And so begins a race against time as Sherman battles to reclaim his formula, restore order to his family, and marry the woman he loves … before all hell breaks loose.