Nuclear tests cause shock-waves that reach Monster Island and start to break the island apart. In Japan, inventor Goro Ibuki, along with his little brother Rokuro and friend Hiroshi, survive an aftershock that drains Lake Kitayama. Returning home, the trio are attacked by a trespasser, an agent for the undersea nation of Seatopia. Lead by Emperor Antonio, the Setopians steal a robot invented by Goro named Jet Jaguar. Awakening their monster god, Megalon, the Setopians command Jet Jaguar to guide Megalon in attacking the surface world. However, when Jet Jaguar breaks free and summons Godzilla into the fray, the Setopians request aid from M Space Hunter Nebula Aliens. A tag team match soon ensues between Godzilla and Jet Jaguar against Megalon and Gigan.