Despite the swelling ranks of the overweight, a significant portion of the population seems immune to obesity. Do some people have a natural ability to maintain a thin physique, no matter how much they eat or how little they exercise? This program explores the phenomenon of effortless slimness and its biological causes. Based on a 1967 experiment involving Vermont State Prison inmates, ten volunteers agree to double their normal intake of calories over four weeks—producing intriguing physiological results. The film also investigates genetic factors, evolutionary influences, and learned behavior associated with zero weight gain, as well as the possibility that a virus could be responsible for some obesity cases. A BBC Production. (51 minutes)