Ghidorah returns
He’s messing with aliens
Godzilla dances

This one is important because it introduces actual freaking space aliens to the proceedings, and also because it’s the one where, as mentioned, Godzilla dances:

This is where the series just gets kind of nuts. “Monster Zero” is also the name the aliens (from Planet X, who are perennial Godzilla foes) gives Ghidorah. I believe Godzilla and Rodan are Monsters One and Two.