It's a sequel movie to both SSSS.Gridman and SSSS.Dynazenon, being a crossover with both series's characters and (of course) kaijuu for them to fight. It's also so bad that it's educational. It's demonstrating why its chosen genre (tokusatsu hero) is fundamentally bollocks, at least as commonly understood in Japan.
Mind you, the film's self-aware about its choices. Two of its main characters (Shou and Rikka) are writing a tokusatsu hero stage play based on the events of SSSS.Gridman for their school culture festival. Sho likes fight scenes and Rikka likes human interest. Which of those two should the genre be about? Shou shows some TV nonsense to Yuuta and says "that's the heart of tokusatsu!", then they go to see a human interest play and that Shou finds boring (even though Yuuta's moved to tears).
This is a real debate. Genre fans discuss it. Unfortunately, Shou's wrong and this movie proves that. A plot summary goes as follows:
1-13 MINUTES: human interest. Shou and Rikka are the only people who can remember the events of SSSS.Gridman, which is toughest on Yuuta since he spent two months transforming into Gridman himself and fighting kaijuu. He doesn't remember any of this. I dislike amnesia as a plot device, especially if I see the start of a story rebooting relationships for the sake of a remake-as-sequel, e.g. Ghostbusters II breaking up Bill Murray and Sigourney Weaver. (That film didn't have amnesia, but never mind.)
13-21 MINUTES: a token kaijuu shows up because... uh, the film's producers wanted an action scene in the first half. (And I suppose it does kick Yuuta into Gridman mode and trigger the arrival of crossover characters.) It's lovingly animated, mind you. What it does to buildings and vehicles is spectacular.
22-54 MINUTES: human interest. This is engaging and likeable. I'm fond of these characters and their interactions, e.g. Yuuta's facial expressions... but one gradually becomes aware that there's almost no narrative element to any of this. The film's genre is kiddie nonsense and its plot is dutifully following in these genre footsteps. The whole thing's building up to giant pointless fight scenes, justified by silly cosmic handwaving that's more interested in the word "multiverse" (spit) than in its heroes' decisions.
54-99 MINUTES: kaijuu fights! This is cool when they play the theme song as stirring incidental music, which is the oldest trick in the book but still worked even for me. Ultimately, though, you can fast forward through all this. It's tempting to do a fan edit that cuts those 45 minutes. It would be a shame to lose the return of Akane, but ultimately there's no meaningful content here. Big thing hits big thing! Repeat a lot. It looks cool, but it's empty. PUNCH PUNCH PUNCH between skyscraper-sized monsters says nothing about the characters and gives them no agency or presence. The only meaningful content here is some bizarre mythology. Gridman is a god who creates multiuniverses, but monsters can in turn control him to seal this universe inside Gridman's body and create kaijuu. Eh? Someone also says that mankind is the only species with the ability to believe fiction.
99-118 MINUTES: back to the humans, thankfully with no sodding kaijuu. This can be charming, e.g. Yume's blush or Yuuta finally confessing to Rikka. It also gets meta again by returning to Shou and Rikka's tokusatsu hero stage play, complete with discussions afterwards of its audience reception. "I don't think they got what I was trying to say."
This movie is broken. Its plot (using the word loosely) is built around kaijuu battles and cosmic nonsense, with no human element. Admittedly the cast are capable of transforming into heroes and robots... but "fight kaijuu" isn't a character-focused action. It's boring. I like the stuff around the edges, but this is a movie I'll be deleting as soon as possible, despite being a big fan of SSSS.Gridman. It's worse than SSSS.Dynazenon.