The plot of Thinner concerns massively overweight lawyer Billy Halleck (Robert John Burke), who is receiving an oral gift from his wife (Lucinda Jenney) while driving down the street one night, when he becomes so carried away that he runs over an old Gypsy woman (Irma St. Paule), killing her. Nobody in town likes the traveling Gypsies much, so Halleck's pals — a judge and a cop — cover up the incident. After the cover-up, the dead woman's father, Tadzu Lempke (Michael Constantine), touches Halleck's face and whispers "thinner." Pretty soon, Halleck is losing weight at an incredible rate of more than 40 pounds a week. He tries everything, but learns that Lempke is the only man who can remove the curse. In the meantime, the judge dies of a mysterious skin ailment, and the cop commits suicide. When begging and pleading with Lempke doesn't work, Halleck turns to more drastic methods of persuasion.