During the Cuban missile crisis in 1962, a Russian submarine strays into American waters. On board is a nuclear cargo destined for Castro. The Americans attack and destroy the sub. 38 years later US submarine Roosevelt is in the same waters. An unknown object attacks and disables the sub with devastating force and drags the vessel to the bottom of the ocean. What lies on the ocean bed beggars belief: dozens of wrecks, among them the sub destroyed 38 years ago. Whatever was on board has fed a creature of unbelievable size and strength! The only way out is the emergency submersible and a passing cruise ship. Above or below the water, there is no escape from the monster mutant octopus with a nuclear diet...

Dead bodies are being found in New York harbor. The police have no clues nor suspects until Nick and his colleague realize the killer is a giant octopus. Everybody, especially the police captain, refuses to believe Nick's story, and soon the harbor will be filled with boats for the 4th of July celebrations.