Murphy stars as the shy Dr. Sherman Klump, a kind, brilliant, calorically challenged genetics professor. When beautiful Carla Purty joins the university faculty, Sherman grows desperate to whittle his 400-pound frame down to size and win her heart. So, with one swig of his experimental fat-reducing serum, Sherman becomes "Buddy Love", a fast-talking, pumped-up, plumbed-down Don Juan. But both the potion and Buddy prove to be uncontrollable, and Sherman discovers he has created a monster! Can Sherman stop his buff alter ego before it's to late, or will Buddy have the last laugh.