While on shore leave, Johnny Drake (Dennis Hopper) meets Mora (Linda Lawson), a woman who spends her time posing as a mermaid on the boardwalk. Her life as a fake mermaid might be more real than either of them think, though. She tells Johnny she believes she's a true siren, like the mermaids of the deep, and Johnny almost starts to believe that Mora's two dead boyfriends were a result of something she'd done. Johnny isn't sure if he should grow wary of the beautiful young woman or not, but when he almost drowns at (what he assumes to be) her hands, he can't help but wonder what's true.