paul marsh has a dream that he discovers a mermaid with razor-sharp teeth while scuba diving into a strange underwater building. 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.