The guardian of the underground kingdom of Seatopia, Megalon was unleashed upon the surface world after repeated nuclear tests had ravaged Seatopia. Megalon surfaced in Japan and attacked Tokyo, guided by the stolen robot Jet Jaguar. However, Jet Jaguar was soon freed from the Seatopians' control and turned against Megalon. Gigan arrived to aid Megalon, but the two monsters were sent retreating after Godzilla joined Jet Jaguar in battle. To date Megalon has only made a single film appearance, but has remained a popular monster nonetheless, appearing in countless Godzilla video games, along with manga and comic books.
He can be seen at the Oasis racetrack as an obstacle battling against Godzilla.