Dreaded Megalon from the undersea kingdom! A miraculous deathblow from the wound-ridden Godzilla!

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.