It's the 2004 Magical Girl Lyrical Nanoha TV series, except good. Well, good-ish. I still wouldn't recommend it, but they've fixed the original's most glaring flaws. However in some ways the TV show was better and it still has the same fundamental problems (e.g. prioritising action over characterisation).
The story's the same. Nanoha is a nine-year-old girl who's recruited by a talking ferret to hunt down magical Jewel Seeds. She'll be doing this as a magical girl, obviously, complete with nude transformation sequence and a "magic wand" that can knock down skyscrapers. She also has a rival, called Fate.
Firstly, they've really trimmed the fat. That running time includes a ton of extra material with Fate's mother in the last act, so you can guess how ruthless they've been with the earlier episodes. This is a good thing. Movie adaptations of TV shows tend to fail through over-compression, but not here. The TV series is boring. Nanoha is a one-dimensional character with no conflict in the show's first few episodes, although things improved later when she acquired a one-dimensional rival.
The movie in contrast takes a butcher's axe to all that. It only keeps stuff that matters, i.e. not a lot. Fate appears as early as the second monster. No one visits a hot spring. Nanoha's family gets no dialogue at all (although we do see them), while even her friends get very little screen time. The latter is particularly important. The TV show damaged itself by putting lots of emphasis on Alisa and Suzuka, only for the supposedly friend-oriented Nanoha (i.e. that's her sole character trait) to ditch them almost immediately and never tell them anything. Alisa even got upset about Nanoha shutting them out, which got resolved with the following moral: "it's okay to stonewall your friends and you don't have to do anything for them to forgive you for it".
Here, though, Nanoha's friends get as much screen time as their plot roles deserve. That's not much. We meet them occasionally. You're thus not upset when Nanoha brushes them aside, because they hadn't been prominent enough for it to feel like much of a brushing in the first place. Alisa still gets angry at Nanoha, but this gets resolved wordlessly and everyone's friends again. (Oddly, this simple forgiveness felt truer to me. It's given freely.)
Besides, movie-Nanoha does tell them something, even if we never learn exactly what she said or how much of the truth she glossed over. It still makes a difference for me, though. I approve.
As for the animation, it's glossier. The 2004 series looks dated, frankly, whereas this often looks very nice indeed. It's a movie! The action scenes are more impressive, the monsters are scarier and the whole thing has pace, punch and (for a while) good momentum.
They've even tweaked continuity to match the later TV seasons. Thus Nanoha's magic wand (Raising Heart) no longer speaks just to deliver status updates, but has become a teacher who explains everything to Nanoha and has quite in-depth, technical conversations with her. This is weird because Nanoha's a nine-year-old Japanese schoolgirl and Raising Heart only speaks English. How does Nanoha understand all that? Magic? There's no explanation. Either there's an unseen magical handwave or else Nanoha is bilingual (despite having a completely Japanese family).
Fate's wand (Bardiche) has a less striking voice, mind you. It's the same voice actor, surprisingly, but I think he was doing a better voice in 2004.
The big change, though, is Fate's mum, Precia. She's still a monster, but they've humanised her. You understand where she's coming from. You can feel sorry for her. The movie's last act is effectively The Tragedy of Precia Testarossa, with Fate and Nanoha being a couple of tag-alongs who happen to be present too. This is interesting and works quite well.
So, what are the downsides? For the most part I approve of the abridgement, but I did occasionally miss the TV show's build-up. Precia doesn't get established as a figure of dread before we've even met her, for instance. She just suddenly pops up. Fate's talking to her, with no lead-in. I prefer the TV show's version of Fate generally, in fact. Fate and Precia are now fulfilling different narrative roles. Movie-Fate thus gets less attention than TV-Fate and I thought what happened to her seemed to have less emotional weight.
Overall, the film's a huge improvement. It works. It's not boring. More specifically it's faster, glossier and more exciting. However it still has the underlying problem of Nanoha and Fate not being very interesting characters, not helped by the story slightly taking its eye off them in the last act to focus on the tragic villain instead. It's a much more competent version of this Nanoha story, but the story itself still isn't great and there are better things you could be watching instead. Personally I think Nanoha suffers by comparison with more recent and far more extreme alternate-demographic magical girl shows. It was big back in the day. It still has a lot of fans. It shook things up and did things with the genre that no one had seen before. However that's not the same as being actually good.