Driven both by fate and betrayal, a pair of hopelessly entangled lovers lose their grip on reality with terrible consequences. A sailor, Johnny Drake (Dennis Hopper) and Mora, a beautiful sideshow performer, fall for each other, yet everyone around them is wary of the romance. Strange events surround the couple and dark secrets lurk in the shadows. Mora believes that she is a descendant of the Sirens, mythic sea creatures who lure sailors to their deaths. These inhuman origins drive her to commit murder by full moon. Johnny is unable to believe his love is capable of such an atrocity, but Mora herself seems more certain of her destiny. When a midnight diving trip gives her the opportunity, Mora is unable to resist the urge to kill. Johnny is forced to choose between his love for the girl and fear for his life and his decision leads to a horrific and unexpected conclusion.