Peter Duck

The Swallows and Amazons are in Lowestoft, preparing for a cruise aboard a schooner, The Wild Cat, with Captain Flint, the Blacketts' uncle Jim Turner. Unfortunately the other adult (Sam Bideford) cannot come and so the cruise is threatened until Peter Duck, an elderly seaman, offers to come along to help. In the harbour a larger black schooner, the Viper is fitting out for a voyage and Peter Duck's presence aboard the Wild Cat interests Black Jake the Viper’s captain. Peter Duck spins a yarn about a treasure that he saw being buried long ago, when marooned on a desert island in the Caribbean Sea, and which Black Jake wants to find. When the Wild Cat sails, the Viper is quick to follow and trails her down the English Channel, at one point threatening to board her in the night.

In a fog off Land's End, the crew of the Wild Cat give the Viper the slip but pick up the Viper’s cabin boy, Bill, who has been set adrift to try and fool the Wild Cat’s crew with false signals. They continue across the Atlantic Ocean to Crab Island where they spend several weeks searching in vain for Peter Duck's treasure.

When a hurricane blows up, Peter Duck and Captain Flint take the Wild Cat out to sea to ride out the storm, leaving the Swallows and Amazons ashore. There is an earthquake during the storm and when the schooner returns, all the paths to the treasure-hunting area are blocked by landslides and fallen trees. However, a fallen palm tree exposes a small box, Peter Duck's treasure, which the children recover. They decide to sail round to the anchorage as the land route is blocked.

While Captain Flint attempts to cross the island to rescue the Swallows and Amazons, the Viper arrives and Peter Duck and Bill are captured. The crew of the Viper also go ashore look for the treasure. The children rescue Peter Duck and Bill and then the Wild Cat sails back to the other side and pick up Captain Flint just before Black Jake arrives. They attempt to sail away from the island but the wind dies and the Viper looks like catching them when they are saved by a waterspout which destroys the Viper. They return home safely without further incident. The treasure proves to be a collection of pearls.