That was both beautiful and emotionally rough. It's one of the best anime films I've ever seen. I'd strongly recommend it, but be warned that being driven to attempt suicide by self-hatred is something that happens more than once in the film.
It's not a depressing film, though, and you won't want to slit your wrists afterwards. It's detailed and uncompromising in its examination of its issues, but the ending is positive and it's capable of being funny.
It's about bullying and one of the main characters is a deaf girl. That's uncomfortable already. Japan has a chronic bullying problem, so certain persons launched legal action to prevent the original manga's publication on the grounds that it showed a negative side of Japanese society. (They failed and everything I've heard about the manga says that it's fantastic.) Disabled people are no exception to the above.
The two main characters are Shouya Ishida (male, about to jump off a bridge as the film begins) and Shouko Nishimiya (deaf, apologises a lot, so determinedly nice that she's turned herself into a doormat). We first meet everyone in elementary school. Nishimiya's joining an ordinary school, instead of a deaf one. People are nice to her at first (although the teacher's beyond useless), but then it becomes clear that she's disrupting the social status quo and some of the other girls get bitchy.
Ishida, though, is the kind of small boy who sees other people as his toys. He bullies Nishimiya, then gets annoyed at her for not getting angry at him. Some things he does are vile. You'd push him off that bridge yourself.
Things get interesting (and almost as nasty), though, when he gets his comeuppance. Everyone assumes he's evil and treats him as badly as he treated Nishimiya, except that it's cold and deliberate and it goes on for years. He gets bullied... and he thinks he deserves it. He agrees with what everyone's been saying. "I'm a horrible human being. I don't deserve to live." He believes everything really was his fault, even though he'd just been one end of a spectrum that also includes "similarly nasty accomplice" and "watched it all happening, never suggested that she had a problem with it and never lifted a finger to defend Nishimiya".
There's a lot of subtle, dark characterisation in this one. Nothing's boiled down to goodies and baddies. There's blame to be handed out on all sides and some occasionally shocking attitudes, even years later. The character analysis goes deep. Being a victim doesn't make you a saint, although that doesn't mean that your problems are your fault. Nishimiya's niceness is deconstructed, since it just irritates her bullies and arguably contributed to the problem since it meant they couldn't empathise with her. The older, bullied Ishida is living in a world of horror (represented in part by big crosses he sees on everyone's faces), but it's one of its own creation and a lot of the social exclusion is something he's doing himself. He could stop doing a couple of key things tomorrow. It's just that he's so damaged that he doesn't think he has the right.
The character animation is also stunning. It's by Kyoto Animation and that alone is a reason to watch. Look at the mum the morning after a big suicide argument (and also a humiliating goof). She's trying to act normal, but you can see she's walking on eggshells. You'll also see a lot of the quiet, passive acquiescence of depression, captured so accurately that you could almost climb inside and feel it.
There's a lot here. It's a two-hour film, but even so no one had expected Kyoto Animation to succeed in telling almost all the manga's story in that time. It's a story about people trying to make amends for something they can't forgive themselves for... but there are also people who can't forgive, or who don't see the point in making the effort. You'll see someone who doesn't think she did anything wrong and then drove someone to the point of suicide. They'll then have a furious go at their victim.
It's a happy film, paradoxically. (It's leading its characters from darkness into light... mostly, unevenly, with a lot of flaws and ugliness along the way.) It's not too heavy going. It's funny, sometimes. Everyone grows and changes. Especially compared with other anime films, it's extraordinary.