In the year 1973, the most recent underground nuclear test, set off near the Aleutians, sends shockwaves as far south as Monster Island, disturbing the monsters, causing Anguirus to fall/burrow into a fault opened up by the consequential earthquakes and Rodan to fly off, while Godzilla decides to stay put. For years, Seatopia, an undersea civilization, has been heavily affected by this nuclear testing conducted by the surface nations of the world. Upset by these tests, the Seatopians plan to unleash their civilization's beetle-like god, Megalon, to destroy the surface world out of vengeance.