What happens at Kattskär?

In the morning Teddy, Freddy, Johan and Niklas discover that they are not alone on Kattskär. They quickly understand that the two strangers on the island are the thieves that steal fishing-nets. Johan sneaks on board the thieves' motor-boat and takes the sparking plugs, so the thieves cannot leave the island. The children throw the sparking plugs into the sea. The thieves try to lay hand on the children's boat, but the children succeed to go away in their boat, leaving the thieves on the island, where they have to stay until the coast guard captures them.