You'll be stumped...
As with most Godzilla sequels featuring more than one giant creature, and they all do, the story is dense with superfluous details and doesn't always focus on the fun stuff. The dialogue is tedious, very procedural and compensates for a budgetary lack. It's as good as any science-fiction film of its time, but the excessive padding is overwhelming, as it turns out.
The humanoid alien subplot can be heavy for someone looking for pure entertainment. Humans negotiate and discuss with them for way too long and display too little natural discomfort. It's hard to take seriously. It is not the first time mankind has had monsters as allies against Godzilla; here Rodan and Astro-Monster, namely, and this comes out as another silly, yet cute attempt at horror-fantasy.
Invasion of Astro-Monster, like all the films in the franchise is an atmospheric masterpiece, and, as such, a delight for the acquired fan. The sound effects, the scores, the sets and the visuals have a particular signature to them that makes you come back for more. Yes, it's more of the same, but with now fully assumed cheese, funny costumes and hilarious oversimplified alien politics.