Seaman Johnny Drake (Dennis Hopper), on shore leave, finds a "Mermaid" sideshow attraction at the marina, operated by Captain Murdock (Gavin Muir). The "Mermaid" Mora (Linda Lawson) and Johnny fall for each other. Everyone around them is wary of the romance, as her previous lovers have died mysteriously. Mora believes she is a descendant of the Sirens, mythic sea creatures who lure sailors to their deaths. From time to time a mysterious and seemingly sinister woman (Marjorie Cameron) appears and frightens Mora. She believes the woman is the leader of the mermaids, calling Mora to fulfill her destiny. Mora's origins seem to be driving her to commit murder by the full moon. Johnny is unable to believe his lover is capable of murder, but Mora herself seems more certain. During a midnight diving trip with Mora, under a full moon, Johnny realizes that people's warnings about Mora may be true, and he swims away from her. When Johnny returns to the marina the next day, he sees that the lifeless body of Mora is now on display in the mermaid tank. Murdock appears, brandishing a gun. It is he who has been committing the murders and convincing Mora that she was an actual mermaid. Mora had discarded her oxygen tank in the sea, and had drowned. In the ensuing struggle between Johnny and Murdock, the glass tank is shattered, and the water and debris pin Murdock down. In the film's conclusion, at the police station, Murdock confesses to the crimes and Johnny returns to his ship accompanied by the shore patrol. The one loose end to Murdock's story is that he denies any knowledge of the mysterious woman who had been frightening Mora. The police dismiss it as the man protecting her, but the door is left open to other possibilities.