Having tried everything–diets, exercise and weight loss experts–in an unsuccessful attempt to shed some of his 400 pounds, Sherman Klump invents a revolutionary fat gene formula. Testing it on himself, he discovers he’s no longer the man he used to be; in fact, he’s only half the man he used to be. With one swig, the gentle, painfully shy academician is transformed into the swaggering Casanova, Buddy Love–an irresistible ladies’ man.

As Buddy Love, Sherman Klump for the very first time, experiences life that he could once only envy. Buddy Love can dish out what kind-hearted Sherman has always had to take. Most remarkable of all, this new personality gives Sherman the confidence to pursue the chemistry department’s beautiful new professor, Carla Purty (Jada Pinkett). Indeed everything, to Sherman’s utter amazement, is beyond his most perfect dreams–until one fateful day when he discovers that his long-lasting creation is actually an unstable substance that can wear off at anytime.