For a long time, a group of friends have been on a yearly summer trip to Laholm. This year, they want to go somewhere else, at least everyone except Ulf, who force the others to join the trip - by saying he has cancer.
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.
Around the time that the chief of police is elected the Most Popular Citizen of Valleby, a thief and impostor is released after two decades in jail and returns to the town with a revenge plan in mind.
