 Söderman urges Nisse Grankvist to report to the police that some thieves are stealing fishing-nets from the fishermen. Johan, Niklas, Teddy and Freddy go on a boat-trip to Kattskär, an island far out in the outskirt of the archipelago, where they are going to stay overnight in a cottage. The island is uninhabited, but there are some cottages, which are used occasionally by fishermen. The four young friends think they are alone on the island. They don t know that the thieves, Berra and Elis, are using the island as their hiding-place.