In 2XXX A.D. when the orbits of Pluto and Neptune exchange their positions, the earth receives a declaration of war from an unknown planet. The invaders from Planet X attack the earth as part of their plan to conquer the entire universe using a legion of space monsters.
The earth sends Godzilla, king of the monsters on the earth, and Mothra, to
defend itself. Godzilla advances from battle stages called Ring Fields
provided in interplanetary space to battle that legion of space monsters
controlled by Planet X. Godzilla must battle its way from one Ring Field to another, and finally defeat Ghidora at Planet X to defend the earth.
For the sake of the future of the earth, Godzilla's fierce battle now
--from the NES Godzilla instruction manual