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.

Katsuhiko Sasaki
as Inventor Goro Ibuki

Yutaka Hayashi
as Hiroshi Jinkawa

Hiroyuki Kawase
as Rokuro 'Roku-chan' Ibuki

Kanta Mori
as Japan Special Defense Forces Chief

Kotaro Tomita
as Lead Seatopian Agent

Ulf Otsuki
as Seatopian Agent

Sakyo Mikami
as Truck Driver's Assistant

Fumiyo Ikeda
as Man from Unit 1

Robert Dunham
as Emperor Antonio of Seatopia

Gentaro Nakajima
as Truck Driver