The movie adheres to the time travel, alternate dimension, and fourth wall breaking plots of the original series. The movie is an extravagant anime with a big cast and an enigmatic atmosphere. Despite the large number of characters, the main plot is absurd but warrants its inclusion.
Despite the fantasy plot, Gridman's characters have always struck me as being more lifelike than those in many other anime. They seem to constantly have the mood of a group of high school students hanging around just right, and they communicate themselves just as much via their actions and mannerisms as through their speech. They flirt, joke about, and tease one other; it comes easy to them. Despite the fact that the action may revolve around robots, the likable and convincing cast is everything but. In relation to robotics...
One of SSSS's main selling points is the mech/kaiju action. In this regard, Gridman/Dynazenon and the movie excel. Gridman's usage of computer-generated imagery (CGI) for its vibrant kaiju and mechs is advantageous since the stomping motions perfectly capture the tokusatsu aesthetic of a man in a suit. The effects are fantastic when combined with a really dynamic, 80s/90s animation style. The first two major action set pieces in the movie add to the television series' already fantastic elements and give them a movie budget and staging. The movie then reaches its climax.
So, if you've already seen it, you'll love this movie, and I can nearly assure you that you'll enjoy it. It can feel bloated when two entire casts of characters crossover, with some being completely left out. But as the movie really gets going, it's difficult not to smile foolishly as we use our Fixer Beam to blast through the universe's horrors.