Steve, Sandra, Ranji, Shirley and Kevin learn from Merric, their friend from a far-off world, that the wizard Maroth is in trouble once again. They return with Merric to help the kindly, yet indomitable, magician. They find themselves involved in a fast-moving, electrifying adventure that becomes far more exciting and frightening than they could have ever imagined.

Travelling in an eerie subterranean land off the west coast of Wales, they come face to face with strange creatures, and encounter many heart-stopping perils, before their quest finally comes to a spectacular climax on Bardsey, the Isle of the Sleeping Wizards, the final resting-place of the greatest wizard of all, Merlin!