This book is set in and around Hamford Water in Essex, close to the resort town of Walton-on-the-Naze. It brings the Swallows and the Amazons together and introduces a new group of characters, the Eels and the Mastodon. Ransome sailed to Hamford Water, an area of salt marshes and low lying islands in his boat "Nancy Blackett". He set the book in this tidal place which offered a new setting for his characters and chances to explore and map the area.