Nuclear tests cause shock-waves that break apart Monster Island, triggering a series of events that threaten the surface world.

Godzilla vs. Megalon
The brilliant inventor creates a humanoid robot, Jet Jaguar, which is then taken by the underwater nation of Seatopia. Seatopians utilize Jet Jaguar to guide their monstrous warrior, Megalon, to wreak havoc on the surface world, seeking revenge for the nuclear tests that have ravaged their society.