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.