Before I watched this I had the impression that it was about how "life goes on even during a war." And that's mostly true, although when your family lives on the outskirts of Hiroshima there's a bit more to it.
We first see Suzu as a young girl, along with her siblings. While her appearance doesn't change significantly, we're then given to understand it's 1943 and she's now 18, and a young man she doesn't know has proposed marriage to her. She accepts, moving from one Hiroshima-adjacent town to another. Her new family (they live with her husband's parents) are mostly very decent people - but they suffer through harsh food rationing and frequent air raids. They all lose family members - not just the brothers who have joined the military, but those whose houses are bombed.
The movie has a huge sadness hanging over it because you know what's coming. "Foreboding" might be a better word, but "sadness" felt right to me. And yet there's always hope. It's an amazingly elegant and beautiful film, that never allows you or its characters to give up.
I've seen "Barefoot Gen," and always refused to watch "Grave of the Fireflies" (both Anime about war-time Japan). This isn't as dark as either of them - and is artistically much better than the problematic "Barefoot Gen."
This is in no way a children's film. It starts out being about a little girl, and it's drawn with gorgeous anime artwork that North Americans often assume make it a children's movie, but it includes arranged marriage, sex workers, the potential of a married woman having an affair, people losing limbs, many people dying, and of course Hiroshima and radiation poisoning. It does all this with somewhat cutesy anime artwork, and avoids looking directly at some things (the sex worker was a sex worker almost entirely by implication), but they're all there.
My two biggest issues with the film are that Suzu never visually transitioned to adulthood, and that no one ever discussed the politics of the war: why are we doing this, should we be doing this? That's a discussion that comes up occasionally in those circumstances, but we don't hear it. With those two exceptions noted, I would recommend this to just about anyone: it's a strange and wonderful - and very beautiful - compound of elegiac examination of war time Japan with ever-present resilience and hope, and the result is both heart-breaking and uplifting.
COVID-19 NOTE: This is a really good movie to watch during the pandemic. Yes, it's somewhat depressing, but I've never seen a movie that gave as much hope, as much belief in the resiliency of humanity.