A trio of high school students, Ichiro, Hiroshi, and Reiko, try to give a demonstration against the suspension , but they are ignored by the students and embark on an unreliable journey.
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.
