Grossly overweight Prof. Sherman Klump, desperate to lose weight takes a special chemical that turns him into the slim but obnoxious Buddy Love.

The grandson of professor Julius Kelp, who created a potion to transform his personality, gets his hands on his grandfather's secret elixir and unleashes his destructive alter ego.

Sherman Klump is an incredibly fat and good-hearted man. He is a college professor on the verge of a breakthrough in DNA restructuring when he meets an admirer of his, named Carla, who is a teacher new to Klump s college. He is enamored of her, but is frustrated by his tremendous bulk. He then decides to test a formula on which he s been working on himself. He is then transformed into the lecherous swinger, Buddy Love, and romantic complications ensue.

Professor Sherman Klump is getting married. And the Klump family could not be more delighted for him. But Buddy Love, his Mr. Hyde alter-ego from the first film, is back and trying to make it on his own. Buddy keeps resurfacing in untimely outbursts, and threatening the portly professor s marriage plans to colleague Denise Gaines. Utilizing Denise s cutting-edge DNA research, Sherman decides to rid himself of his monstrous nemesis -and his disruptive outbursts-once and for all by extracting Buddy s DNA from his system. But Buddy bursts full-bodied into Sherman s world and lays claim to the professor s astounding invention - a revolutionary youth serum. Desperate to keep it from Buddy, Sherman hides the serum in the Klump family home, thinking it will be safe. Buddy correctly divines where Sherman has placed the serum, but to get it, he has to deal with the entire Klump family first.