What is wrong with nudity? Why are people embarrassed about their bodies? How and why did they get the way they are? Horizon takes a group of volunteers and subjects them to a series of psychological and physical tests to challenge attitudes to the naked human form. The questions raised strike at the heart of human physical and social evolution. Human beings are the only creatures that can be 'naked' - but why, how and when did people lose their fur? That question takes Horizon around the world to meet scientists from Africa to Florida, and they are finding answers in unexpected places: the chest hair of Finnish students, the genetic history of lice, and the sweat of an unusual monkey. It turns out that something everyone takes for granted may hold the key to the success of the entire human species.