The Walker children (John, Susan, Titty and Roger) who are staying at a farm near a lake in the Lake District of England which is mostly a combination of Windermere and Coniston, during the school holidays. They sail a borrowed dinghy named Swallow and meet the Blackett children (Nancy and Peggy), who sail a dinghy named Amazon. The Walkers camp on an island in the lake while the Blacketts live in their house nearby. When the children meet, they agree to join forces against a common enemy - the Blacketts' uncle James Turner whom they call "Captain Flint" (after the character in Treasure Island). Turner, normally an ally of his nieces, has withdrawn from their company in order to write his memoirs, and has become decidedly unfriendly. Furthermore when the Blacketts let off a firework on his houseboat roof, it is the Walkers who get the blame. He refuses even to listen when they try to pass on a warning to him about burglars in the area.