The alien Zenon have come to invade Earth in a star destroyer they stole from Star Wars. Luckily the Earth is defended by three female super heroes. Unluckily, they have no weapons or useful strengths and if they transform into their outfits, they will be shot from space. The Zenon’s plan is to set loose a bunch of monsters that happen to be the one in previous Gamera movies. Of course Gamera shows up to defeat the monsters just as he did before. Also hanging around to use up time is a kid who plays the Gamera march on the organ and seems to have a pointless connection to Gamera and an evil Zenon lady who isn’t quite up to the task of dealing with a child.
Daiei was dead, its assets bought by another company, which decided it wanted a Gamera movie, though they didn’t want to pay much. They weren’t even a film company, but figured they could make something back on their investment. The old contracts were still in force, so the filmmakers had no choice. And it turns out a giant monster movie is pretty cheap if you don’t film giant monsters. So they edited together clips from the previous films and shot a minimum amount of new footage, mainly dealing with humans in everyday environments, and they had their film.
The actions of the humans (and space women) have nothing to do with the monsters’s actions. People in the streets don’t act like the country is under attack (I’m guessing many weren’t aware they were in a film). There’s no attempt to make the pieces fit together. The evil lady wants to kill the superhero women because they are somehow on Gamera’s team, except they aren’t in any way. They just watch him on their home video screen. The voice from the space ship keeps threatening the evil girl when Gamera wins due to her failure, except she’s not in charge of the battles with Gamera—she came down to get the three superheroes. Gamera: Super Monster is just some nonsensical new clips stuck between old footage.
Is this worse than the abysmal Gemera vs Zigra? As a movie, yes, much worse, as it barely qualifies as a film. However, if all you want is a way to catch up on the bad Gamera movies, perhaps for your geek trivia contest, without having to watch those films, then this serves a purpose.