Ben and Tim and their cousins Thomas and Meg don't expect unusual adventures when they begin their summer holidays at the beach. But they couldn't be more wrong! When they stumble across a mer-boy called Sechiel stranded on the sand, they hear about a sinister diver in a submarine, who together with his razor-toothed shark is terrorising the mer-people and destroying the wildlife. Worse still, he has abducted Sechiel's sister! All of a sudden, Ben and Tim find themselves in the middle of their most exciting and magical adventure yet.