Goodness, it really is Sabu. It seemed improbable, but I've just double-checked and it is indeed him. He's not even directing someone else's screenplay.
The explanation is that he's adaptating a novel.
Hitherto a Sabu film had been violent and funny. There would be lots of physical movement and little introspection. His taciturn protagonists would rarely reveal much about themselves, but over the course of their extreme journeys would go through a process of self-discovery. Sometimes this would kill them, of course. Anyway, these films would be deeply unconventional in their plot development, in a blood-stained crime-filled manner that should appeal to fans of Quentin Tarantino or Guy Ritchie. I like them a lot.
This film, on the other hand, is conventional by the standards of the Japanese movie industry. I can see the links with Sabu's earlier work, given its taciturn protagonist and themes of crime and violence, but they've been turned on their head. These yakuza aren't the heroes. Crime isn't a world you live in. On the contrary, we're following a little boy as he grows up. It's about ordinary people. The film's interested in the effects of crime and violence, including people's sometimes ugly reactions to it, whether or not their suspicions are founded in truth.
It's a slow film, in which not a lot happens for long stretches and we're simply watching the characters change, not always for the better. Sometimes there's even narration, because just watching everyone go about their lives would have been too minimalist. Our hero, Yuya Tegoshi, is normal. He's not chatty, but he has typical parents and a big brother who likes to know all the answers. They live in a small community near the sea, about which the most notable thing is that there's a nearly identical community nearby which it's said they look down on. There are occasionally yakuza, but Tegoshi should be safe enough since even yakuza don't normally go around attacking children. There's a girl. There's a priest, about whom people say things and whose congregation contains only one person. (How does he eat? I presume he gets a church stipend of some kind, but even for a man of the cloth that looks like a remarkably quiet existence.)
However there's also violence, none of it cool or happy. People have committed murder or suicide, or else regret not having done the latter. We talk to a man on Death Row. Tegoshi's teacher isn't particularly pleasant, although he's less of a bully than some of his pupils later on when things turn bad.
This is explored in detail. There's guilt and grief. People say they want to die, or else wish they'd killed themselves before something bad happened. However I don't want to give the impression that this is depressing or a gloom-fest, because it's not. It's a long way from being light-hearted, but that's not the same thing. I didn't find it hard going at all, instead finding it interesting, with a strong exploration of its themes. Basically it's everyday life with dark undercurrents, except that those undercurrents are clearly what the film's talking about.
Act Three changes the rules a bit, mind you, but you were expecting that.
Anyway, this kind of thing is exactly what you'd expect from a serious-minded Japanese movie. Quiet, thoughtful minimalism that's going to bore most people. Sabu does it quite well, but the surprise is that he's doing it at all.
This film marks a point where you could say he stopped making Sabu films. After this he took a disillusioned four-year break from the industry and didn't return until The Crab Factory Ship, which again he'd adapted from a novel instead of writing an original screenplay. Furthermore the original was written by a communist in 1929 about exploited workers and had already been turned into a movie in 1953. Returning to the subject of this particular film, I thought it was pretty good. It is what it is. It sets specific goals for itself and meets them. It's the kind of self-contained artistic achievement that isn't going to appeal to everyone, but clearly knew what it was trying to do and, within those drab parameters, is more or less flawless.