It's a charming little SF fantasy. It starts deceptively, though, pretending to be an ordinary, realistic tale about two schoolfriends, Shouma and Atsushi. If you think you might watch this, then I'd suggest not reading this review and jumping into it cold.
NOT BIG SPOILERS
BUT THEY'RE STILL SPOILERS
IN A GENTLE WAY
EVEN IF WE'RE ONLY TALKING ABOUT THE FIRST TEN MINUTES
Okay, that'll do.
Shouma and Atsushi used to be best friends, but that was three and a half years ago. They haven't seen or contacted each other since... until suddenly Atsushi contacts Shouma to invite him to a festival. Shouma's astonished, but he's also very keen to catch up.
On trying to follow Atsushi's directions, he ends up in the mountains with his compass spinning and no sign of another human being.
Shouma gets rescued by a girl called Shiori and a slightly translucent hologram called Kanna, which may or may not be connected with some glowing purple magical blob-lights. He learns that the Starlight Festival is strict about: (a) uninvited guests, (b) personal possessions, and (c) not letting anyone use their smartphone.
We learn what the Festival is, although the mechanism by which is happens is never entirely clear. Fortunately it's also not very important. Is it magical? Is it SF technobabble? What matters is the emotional weight of what they're trying to do on the last day, which is delicate but considerable. Its nature changes a little with an SF explanation, making it less literal and more of an artificial healing process, but that doesn't invalidate it at all. For Shouma, he'll be understanding the consequences of a huge but well-meaning lie.
The ghost samurai don't work, but they don't damage the film either. They're there. That's it, really, beyond complicating the worldbuilding. They give the animators something exciting to draw and make the film less talky, but you could cut their scenes and never notice. Fortunately, they're not in it much.
There's a lot to like in this film. It's a gentle, mystical little piece, keeping romance at arm's length while it establishes a likeable friendship. It wouldn't be silly to call it beautiful. The emotional vignettes on the last day are strong, even if one could argue that it's surprising that everyone reached acceptance so easily. Well, maybe they'd all prepared properly for it. The communal nature of the experience probably had an effect too.
It's very nice. Recommended to anyone who's looking for some emotional depth.