Percy's gym shoes are like no others. When he has them on, he can do exactly as he likes. Ulf wants these shoes, so that he also can become strong and courageous. The magic shoes are mouldy and smelly, and Ulf pays for them with his steam machine, his stopwatch, his stamp album... This is a fast-paced story of the unlikely friendship that builds between Percy, the school tough guy, and Ulf, eccentric, chubby and sensitive.