The underground kingdom of Seatopia sends out Megalon, a giant beetle, and Gigan to destroy the above ground dwellers. In an attempt to stop them, an independently thinking robot brings Godzilla into the fight.—Todd A. Bobenrieth
An inventor creates a humanoid robot named Jet Jaguar that is seized by the undersea nation of Seatopia. Using Jet Jaguar as a guide, the Seatopians send Megalon as vengeance for the nuclear tests that have devastated their society.