The Swallows and Amazons are sailing with Nancy and Peggy's Uncle Jim (better known as Captain Flint) when their hired deckhand tells them a tale of his younger days - a tale to set pulses racing and hopes shooting sky high. Soon their boat is on its way to a Caribbean treasure hunt and they find themselves up against shark, storm, earthquake - and the vilest pirate who ever eavesdropped at a porthole.