The Walker children (John, Susan, Titty and Roger) are staying at a farm near a lake in the Lake District of England, 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 Jim Turner whom they call "Captain Flint." The book relates the outdoor adventures and play of two families of children. These involve sailing, camping, fishing, exploration, and piracy.
