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
Katsuhiko Sasaki
Hiroshi Jinkawa
Yutaka Hayashi
Rokuro 'Roku-chan' Ibuki
Hiroyuki Kawase
Japan Special Defense Forces Chief
Kanta Mori
Lead Seatopian Agent
Kotaro Tomita
Seatopian Agent
Ulf Otsuki
Truck Driver's Assistant
Sakyo Mikami
Man from Unit 1
Fumiyo Ikeda
Emperor Antonio of Seatopia
Robert Dunham
Truck Driver
Gentaro Nakajima
Antonio's Aide (Radio Operator in White)
Ralph Jesser
Godzilla
Shinji Takagi
Megalon
Hideto Odachi
Jet Jaguar
Tsugutoshi Komada
Jet Jaguar
Masachika Mori
Gigan (as Kengo Nakayama)
Kenpachirō Satsuma
Emperor Atonio of Seatopia (voice)
Gorō Naya
Narrator
Eisuke Nakanishi
Godzilla (archive footage) (uncredited)
Haruo Nakajima