A swimming machine turns up that has been doctored and modified into a tile thrower, but the machine malfunctions and causes an accident that imparts Pettson with amnesia. Suddenly, it seems as though the only hope for Pettson lies in old stories that will jog his memory, such as the tale about Findus getting lost as a kitten and the one about Pettson and Findus embarking on a thrilling treasure hunt.