Young John Trenchard lives with his aunt in the village of Moonfleet half a mile from the sea. His peaceful existence ends dramatically when he discovers a secret passage into the vault of the powerful Mohune family, who once had control of Moonfleet. Wild stories abound of blood-curdling cries and horrific deaths, for here lies the notorious Blackbeard who, it is said, haunts the churchyard in search of a fabled but ill-gotten diamond. Yet instead of glittering treasure Trenchard finds cargoes of contraband. Drawn into the dangerous, secretive world of the smugglers it is not long before he is forced to flee the country.