Nuclear testing unleashes mayhem on the undersea kingdom of Seatopia. To exact revenge, Seatopia unleashes Megalon, a gigantic beetle with the ability to fire ray beams and napalm bombs.

Nuclear testing unleashes mayhem on the undersea kingdom of Seatopia, causing a series of environmental disasters that nearly wipes out Rokuro, the schoolboy protagonist at the center of this film.

To exact revenge, Seatopia unleashes Megalon, a gigantic beetle with the ability to fire ray beams and napalm bombs. Meanwhile, Rokuro’s brother creates Jet Jaguar, a flying robot with a built-in moral compass. The inevitable matchup of Godzilla and Jet Jaguar versus Megalon and Gigan decides the world’s fate.