Zena is the secret lover of a married woman named Karen Wickman. One day, after discovering how much of an unscrupulous individual she is, a scientist and a military general meet with the woman and offer her an enormous amount of money if she'll kill Karen's husband, Arthur. Zena gladly accepts, and begins to conspire with her lover so that they can inherit the money once he's gone.
Some time later, Zena, Karen, and Arthur (who knows nothing of his wife's affair) take a cruise on the wealthy man's yacht. Using a "special poison" given to her by the scientist, Zena mixes it into a cocktail and serves it to Arthur. His heart stopped, the two women believe him to be dead, and toss his body into the water. What Zena did not know, however, is that the substance was not a poison at all, but a chemical that can transform a human being into an octopus that could possible have potential as a military fighter.In his new tentacled form, Karen's husband climbs back onto the yacht to find the two deadly schemers celebrating intimately from their plan having been such a success. Furious, Arthur executes his revenge in multiple stages, first by unleashing an assault of octopuses and crabs at the attempted killers, and then invasively tearing Zena apart with his tentacles, before moving to eliminate his wife. Upon her own death, Karen is appalled to learn that she, too, was only being used by Zena, who she thought had loved her.