Tragedy is draped over Nobody Knows like a mist. You get the sense from the outset that something isn't quite right, that we're building towards the horrible. And it's full credit to director Hirokazu Kore-eda that he still makes his film so engaging - you don't want to see what's going on, but won't be able to tear your eyes from the screen. This is human drama played out in a relentless downward spiral.

The film's basic concept is drawn from actual events - what came to be known as 'the affair of the four abandoned children of Nishi-Sugamo' - although he's apparently changed events somewhat. We have a single mother Keiko (You), who arrives at her new flat with her 12-year-old son Akira (Yuya Yagira) in tow. What the landlord doesn't know is that Keiko has three more kids hidden away in her luggage. Four kids, from four different fathers - something that she needs to keep out of sight if they're to find a home.

Of the children, then, only Akira is allowed outside - 10-year-old Kyoko (Ayu Kitaura), seven-year-old Shigeru (Hiei Kimura) and four-year-old Yuki (Momoko Shimizu) must remain in the flat at all times. With Keiko away at work - and at play - for most of the day, Akira takes on the role of parent, guiding his siblings through their daily chores and generally looking out for their well-being. And things look OK - just - for a time.

The mother is a flake but shows so much genuine affection and care for her kids when she's with them that Kore-eda doesn't allow us to really hate her. But one day she leaves a note and some cash, saying she'll be away for some time and so Akira must keep the other kids in line, keep the world at bay while their little lives go on. But we know it's only a matter of time before their luck, and their cash, runs out.

Kore-eda allows things to slowly fall apart around the resolute Akira and from Yagira, an untried actor like the rest of the children before this production, he's drawn a remarkable performance. He portrays such strength and dignity in a captivating turn for which he justifiably picked up the best actor award at Cannes this year, despite the moanings of Tony Leung about his age and lack of experience.

It's grim but gripping viewing.

Nobody Knows opens today.