I think the first reviewer misses the point of Kore-Eda's work. He has an almost documentarian's way of showing human behavior. It's decidedly not theatrical. His characters are flawed, real people. Soza-sensei's abhorrence of violence is not undercut by his fear of being sliced up. He discovers the strength to be a real human being amid what he comes to recognize as counterproductive posturing.
The 47 Ronin subplot is not a distraction. It's a mirror of Soza's choosing life over revenge. Susumu Terajima hits just the right note as the surviving Ronin. If you don't know Japanese history and myth-making, it might seem like a distraction, but it is the point of the whole story, that a slavish devotion to the ideals of Bushido is in conflict with living an authentically human life.
Besides, Rie Miyazawa is a total hoot in the play within a play. It is a pleasant surprise that Kore-Eda can do comedy and still keep it real. I was beginning to think that he was only good at grimness, but I was reminded of the small comic touches in "Nobody Knows" and "Afterlife." I can't remember if there was anything funny in "Mabarosi." I just remember how depressing it was.
