It's pretty good. It's exactly like the parent TV show, but that's a strong template. The only strike against this film is its lack of surprises, as it hits exactly the beats you'd expect. It's a mini-arc of consequence-free filler, efficiently doing what a spin-off movie has to do.
PLOT SUMMARY: Midori and All Might go to an artificial superhero research island and Midori's classmates are there too! (Well, most of them. Frog Girl couldn't make it. Tch.) There's a bit of tournament stuff! Irritable Boy is irritable! Supervillains show up and incapacitate all the adults, leaving only our Superhero School fledgling heroes to save the day!
None of that makes it bad, though. I like the wrinkle that our heroes aren't allowed to fight supervillains directly because they haven't graduated yet and it's against school rules. I like the partial focus on how people without superpowers can make a difference too. (Midori used to be powerless, so this resonates for him.) Above all, though, I like the film's heart. This franchise has always been strong on the meaning of heroism. Its heroes want to do the right thing and protect people. That's not just platitudes, either, but something that keeps getting explored, tested and demonstrated. I find it a warm franchise, with more emotional weight than I get from most superheroes.
The film begins with an all-action dream sequence, in case the audience might have got restless waiting for the fight scenes in Act Three. That said, though, these scenes also serve the secondary purpose of showing us All Might's past, when he was a young superhero in America with a muggle scientist sidekick, David Shield. (We'll soon be meeting David and his daughter Melissa in the present day. They're important. They don't have superpowers, but that's integral to what the film's saying.)
There's comedy. Surprisingly, the funniest character isn't the hair-trigger Bakugou, but the craven, pathetic Mineta who's torn between his cowardice and his libido. I did wonder if there wasn't a plot hole when all the students were sticking together in Act Three, instead of letting Iida run ahead with his super-speed while giving Melissa a piggy-back... but in fairness they met plenty of obstacles that speed alone couldn't solve. Jirou's computer-hacking earlobes make her the team guide, for instance.
I saw this in a Japanese cinema, by the way. Mirai no Mirai and Penguin Highway looked like better films, to be honest, but they'll probably get English-language DVD releases eventually and I'll be buying them. This film... not so sure. Anyway, my questions were "is this just a TV series compilation?" (answer: no, it's all-new) and "where does it fit?" (answer: between seasons 2 and 3, which was perfect for me). On presenting my ticket at the door, I received a mini-book called "My Hero Academia vol.0: Origin." That must be one of those freebies you get in Japan to encourage geek collectors to get off their arses and go into cinemas instead of just downloading everything and/or waiting for the Blu-rays. The book's mostly just character outlines, but it ends with a behind-the-scenes text feature and a nine-page manga story called "All Might Rising". It feels like such a fundamental part of his origin story that I'm a bit surprised they didn't put it in the main manga somewhere.
Do you need to watch this film? No. I wouldn't really recommend it for newbies, although superheroes are so simple and universal that I also can't imagine anyone having trouble with it. This is a poor introduction to the series, though, dumping in the cast without re-introducing them. Even if you're already a fan, though, it's just a standalone movie between two TV seasons and you don't need to see it. It doesn't really matter and it's doing nothing you couldn't have guessed anyway. However it's also quite good. It's warm, it's funny and the only thing I disliked about it was Not Enough Frog Girl.