In the Florida Everglades, marine biologist Dr. Richardson and his two aides, John Hoyt and Egon, are conducting experiments with Portuguese men-of-war. Unknown to his associates, Egon, who suffers from a persecution complex because of his horribly disfigured face, has built an underwater cave laboratory in the hope that secrets learned from his studies of the Portuguese men-of-war might help to erase the scars that are driving him to madness. Shortly after the arrival of Richardson's daughter, Karen, and four of her college friends, Egon's tests produce disastrous results, and he is transformed into a vengeful half human, half Portuguese man-of-war. Because he has learned to communicate with the men-of-war, he amasses a host of them and systematically attacks boats and their passengers; and he murders all the students except Karen, whom he loves. Eventually he kidnaps Karen and takes her to his underwater laboratory. John follows but is overpowered by Egon. Before Egon can complete his task, however, he dies tragically as a result of an explosion of his laboratory apparatus.