[2] 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.

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 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.

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 children rescue Peter Duck and Bill, and then the Wild Cat sails back to the other side and they pick up Captain Flint just before Black Jake arrives.