What a classic.
To recap...a new planet gets discovered. It's called "Planet X". There's a monster on the planet that the inhabitants want destroyed. The inhabitants look like Japanese with funky shades.
They sign a peace accord with us (planet Earth) to "borrow" Godzilla and Rodan to destroy Monster X. Er, I mean Monster Zero.
Trivia: Nick Adams plays the token US actor in this. 2 years after his Oscar nomination.