STING OF DEATH

A young blonde woman in a black bikini lies on a dock in Florida, sunbathing and listening to her transistor radio. She turns it off after hearing a news report about unexplained local murders. Then, without warning, a monstrous hand reaches up from the water and grabs her ankle, dragging her under, where she drowns.

Meanwhile, five of six college friends arrive at the home of Dr. Richardson (Nagel), the father of one of them, Karen (Hawkins). Richardson is a marine biologist who studies the "evolution of sea life" with his assistant, Dr. John Hoyt (Morrison). Karen and the others—Louise (Sandy Lee Kane), Jessica (Deanna Lund), Donna (Lois Etelman), and Susan (Blanche Devreaux)—are staying at Dr. Richardson's house. But they all wonder why Ruth (Judy Lee), who had gotten there earlier, hasn't joined them on the patio for a cold glass of orange drink.

Several raucous college students who have been invited to a pool party show up by boat. They taunt Richardson's mildly facially disfigured helper Egon (Vella), who is secretly in love with Karen. Unfortunately for him, Karen has fallen for John. But Egon can take revenge for all the wrongs done him by turning himself into a half-man half-jellyfish monster. He picks off the girls one by one: Ruth was the woman killed while sunbathing; Louise is seriously injured in an attack in the pool during the party; Donna dies while being chased through a swamp; Jessica is killed during the search for Donna; and Susan is murdered while having a shower. Many of the other students die when their boat sinks—the monster has chopped a hole in its hull with an axe—and they are attacked by a school of Portuguese man o' war jellyfish.

Egon forces Karen to swim to his secret laboratory in an underwater cave. There, he shows her an enormous Portuguese man o' war in an aquarium that is connected to the machine that he uses to transform himself from man to monster. John fatally injures the now-transformed Egon during a fight and rescues Karen. Just as she and John surface, the machine explodes, finishing off Egon and his evil alter ego.

Egon, transformed into the jellyfish monster, is fatally injured during a fight and his machine explodes, killing him and ending his reign of terror.

The film concludes with the destruction of both the monster and the means of transformation, restoring safety to the group and the Everglades. The ending reinforces classic creature feature morality: tampering with nature and letting vengeance run wild ultimately leads to the downfall of the would-be monster.