A sea battle in 1683 between a Spanish galleon and a smaller French corsair results in the galleon being sunk, its treasure stolen and the Spanish captain and two ladies taken prisoner for ransom. Later a storm howls for three days and pushes the corsair into a large cave where a landslide removes all trace of it from the world outside. Three hundred and thirty-five years later the Franks twins, on holiday from their home in Norfolk to their aunt and uncle's on the south coast of Cornwall, find a gold coin and a gold cross with an engraving on the back. The two turn detective to see how their discoveries made their way to Cornwall and find themselves in the middle of a centuries-old mystery of the fate of the Sea Hound.