1960s thriller starring Dennis Hopper as a sailor who falls for a woman who believes she is a mythical creature. During his shore leave, Johnny (Hopper) meets Mora (Linda Lawson) when she is performing as a mermaid at a local carnival. Johnny is warned by others of Mora's former lovers who died under mysterious circumstances. Mora believes that her ancestors were Sirens who lured seamen to their deaths. Johnny refuses to think this possible but when the two go diving, she makes what appears to be an attempt on his life. Could she be the murdering creature she thinks she is?