Stock market tycoon Paul Marsh has a dream that he discovers a mermaid with razor-sharp teeth while scuba diving into a carven pit. Paul awakes on a boat off the shores of Spain, where he is vacationing with his girlfriend, Barbara, and their friends Vicki and Howard. A sudden storm blows their boat against some hidden rocks. Vicki is trapped below deck and Howard stays with her while Paul and Barbara take a lifeboat to the nearby fishing village of Imboca. During their absence, an unseen creature from the deep attacks the two in the boat.