Three friends, Madge, Skip, and Coop, rent a boat on a coastal town for a few hours, driven by a deaf man named Hoskins. While on the boat, something hits their boat, damaging the propeller, leaving the group stranded. Madge, Skip, and Coop decide to swim to a nearby island while Hoskins fixes the propeller. When they arrive on the island, they find an old house on the beach, and decide go to inside. After they enter, they are found by a mad scientist named Klaus who lives in the house. He holds them at gunpoint, and explains that in World War II, unauthorized experiments took place on humans, turning them into Frankenstein-like monsters although the experiments were caught, and put to an end. After World War II ended, several groups collected the remnants of the experiments, and Klaus has stolen a brain, and heart from one of the monsters, planning to put it into an experimental shark he's created. He forces Madge, Skip, and Coop to help him, promising he'll let them go once he's done.Madge, Skip, and Coop reluctantly assist Klaus in transplanting the monster's heart, and brain into the shark, before Klaus releases the now-alive shark into the water. He plans to communicate with it using technology he's created, using it to terrorize the surrounding waters. He then locks Madge, Skip, and Coop in a cellar, before using the shark to kill Hoskins. The shark ignores Klaus' commands, instead heading back to the pool where Klaus released it into the water. Klaus attempts to put the shark under control, although it kills him instead. The shark rams the building multiple times, causing the building to collapse, and releasing Madge, Skip, and Coop into the water. Skip is eaten by the shark while Madge, and Coop are rescued by harbor patrolman Duke Lawson, who's been investigating the recent deaths caused by the shark.Duke takes Madge, and Coop to the marina, and the shark leaps onto the dock, allowing it to be struck by a lightning bolt that causes it to grow legs, and giving it the ability to breathe out of the water. The shark runs off into the woods nearby, and Madge, and Duke insist on killing it while Coop wanders off in frustration. The shark then eats several cows at a nearby farm, angering a farmer that creates a mob of locals who attempt to kill the shark. The shark then eats Coop, as well as a porn star named Bonnie Boom Boom, and her cameraman, and a few members of the mob while Madge, and Duke head to a nearby church where munitions are stored, hoping to blow up the shark.Madge, and Duke set up the explosives in the church, and Duke lures the shark inside the church while Madge hides. The mob arrives, and sets the church ablaze despite Madge's objections. As the shark, and Duke both burn to death, the leader of the mob shoots the explosives, killing Duke, and the shark. Madge comments that the heart, and brain of the shark will never be destroyed before walking off onto the dock, where another shark monster lunges at her.