Inventor Goro Ibuki creates a humanoid robot named Jet Jaguar. It is soon seized by an undersea race of people called the Seatopians. Using Jet Jaguar as a guide, the Seatopians send Megalon as vengeance for the nuclear tests that have devastated their society.
Inventor Goro Ibuki
Hiroshi Jinkawa
Rokuro 'Roku-chan' Ibuki
Lead Seatopian Agent
Emperor Antonio of Seatopia
Gigan (as Kengo Nakayama)
Emperor Atonio of Seatopia (voice)
Godzilla (archive footage) (uncredited)