IT'S ALL UP TO A 12-YEAR-OLD.

Byline: Bob Strauss Film Critic

FOREIGN kids-with-problems movies so consistently turn heartwarming or maudlin that it's bracing to see one, Hirokazu Kore-eda's ``Nobody Knows,'' with a sense of behavioral realism that never wavers.

For nearly 2 1/2 hours. Which creates its own problem, one of borderline tedium. Despite that, I'd much rather sit through a long, sometimes repetitive movie that illuminates the lives of children in intelligent detail than one that just tries to wring easy tears out of me. This story, director Kore-eda understands, doesn't need cheap manipulation to earn its sad effects.

A Japanese documentarian who has gained quite the international art- house reputation with such features as ``Distance'' and ``After Life,'' Kore-eda based ``Nobody Knows'' on a true incident. In the movie, a childish, promiscuous woman, Keiko (played by one-word-named media personality You), moves from Tokyo apartment to apartment, sneaking in three of the four children she's conceived from different fathers.

Affectionate - at least when she's around - Keiko delegates most family-maintenance responsibilities to stoic, 12-year-old Akira. Played by Yuya Yagira, in a performance that rightly won the 2004 Cannes Film Festival's best actor prize, Akira is smart (though, like his younger sisters and brothers, he's never attended school), even-tempered - and very much a 12-year-old boy.

This becomes apparent after Keiko takes off on one of her periodic trips with a new man. Only this time, she doesn't return for months. As the money she left for the children's expenses dwindles away, Akira tries to cut corners, bargain-hunt, even hit up his siblings' deadbeat dads. All the while, he struggles to remain a loving big brother to his sensitive sister Kyoko (Ayu Kitaura), boisterous brother Shigeru (Hiei Kimura) and adorable little Yuki (Momoko Shimizu).

But Akira also wants to hang with pals, play baseball and explore the stirrings of puppy love with a truant schoolgirl (Hanae Kan) who takes an interest in the siblings' plight. You know, all the stuff he should be doing at his age instead of constantly trying to keep the other kids hidden from the landlord or figuring out how to get their increasingly ratty clothes washed after the water and power are turned off.

Yagira's is a magnificently controlled performance, its desperation achingly counterweighted by a child's natural innocence, optimism and ignorance of consequence. The other young actors, most of them complete amateurs, are also consistently note perfect in less-developed but always persuasive roles.

Kore-eda combines his objective documentarian's eye with a knack for staging in both cramped quarters and public spaces. ``Nobody Knows'' has a haunting quality that is all the more effective for its deceptively everyday appearance, in which the particulars of tiny lives falling apart accumulate horribly, enragingly - and with all the prosaic inevitability of kids messing up their rooms.

Bob Strauss, [phone]

bob.strauss(at)dailynews.com

NOBODY KNOWS - Three stars

(PG-13: children in jeopardy, language. In Japanese with English subtitles.)

Starring: Yuya Yagira, Ayu Kitaura, Hiei Kimura, Momoko Shimizu, You.

Director: Hirokazu Kore-eda.

Running time: 2 hr. 21 min.

Playing: Laemmle Playhouse 7, Pasadena; Laemmle Royal, West Los Angeles; Edwards South Coast Village 3, Costa Mesa.

In a nutshell: Stark, unsentimentalized study.

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Yuya Yagira is the preteen who must take care of his three siblings when his mother takes off in ``Nobody Knows.''