In the year 1973, the most recent underground nuclear test, set up near the Aleutians, sends shockwaves as far south as Monster Island. For years, Seatopia, an undersea civilization, has been heavily affected by this nuclear testing conducted by the surface nations of the world. Seatopians plan to unleash their civilization's beetle-like god, Megalon, to destroy the surface world. An inventor called Goro Ibuki, his nephew Rokuro, and their friend Hiroshi Jinkawa when agents of Seatopia who are trying to steal Jet Jaguar put under the control of the Seatopians and is used to guide Megalon to attack Tokyo Jet Jaguar and sends the robot to Monster Island to bring Godzilla to fight Megalon and The Seatopians learn send out a distress call to the Nebula M aliens to send Gigan to assist them.