Was there ever really any doubt?
I love this movie. It was the first movie I ever posted a clip of on the Cloud. It's got all the greats: Godzilla, his son (daughter?) Godzookie, who blows smoke rings, Rodan, Mothra, King Ghidrah. Eleven monsters in all! Gamera, sadly, is not in it, as the Godzilla and Gamera universes do not intersect.
The plot: all the word's monsters are rounded up and sent to Monster Island. Those zany Japanese Space Babes let them loose to destroy Earth, and control them from the moon. After their moonbase is destroyed, the Babes unleash King Ghidrah, but the Earth monsters unite to conquer Ghidrah and save the day! Yay!
The movie uses a lot of stock footage from other movies, but my guess is if you're a fan of Japanese monster movies, you're probably not a big stickler for continuity.