Some critics have united the disparate directors of the “New Japanese New Wave”—Takashi Miike (Ichi the Killer), Kiyoshi Kurosawa (Cure), and Takeshi Kitano (Zatoichi)—according to their irreverence: their pervasive interest in genre-busting, their indulgent attitude toward extremely nasty sorts of pleasure and pain, their Godzilla-sized conviction that annihilation is just another way to have fun. Yet one of the group’s most prominent members is Hirokazu Kore-eda, whose austere mourning song “Maborosi” and mortality comedy After Life have set him apart as a sober, socially conscious artist with a fierce resistance to mainstream conformity.

It shouldn’t be surprising that Kore-eda’s Nobody Knows—about a quartet of underage siblings left to fend for themselves when their mom goes off to work one day and never comes back—would prove yet another departure for the director. Though still grounded in Kore-eda’s extensive documentary background and suffused with an almost vérité attention to all manner of spontaneous behaviors and naturalistic details, Nobody Knows is radical precisely for being so emotionally accessible, so thoroughly unashamed of its altogether mainstream appeal. It’s the least likely movie Kore-eda could have made: fearlessly, even recklessly melodramatic at heart.

Based on a news item the director came across more than 15 years ago, Nobody Knows opens as single mother Keiko and her 12-year-old son Akira (Yuya Yagira) are moving into their new apartment. Little does their landlord realize that there are two more children packed inside his new tenants’ luggage and a fourth waiting at a nearby train station to sneak in under cover of darkness and round out their family of five. Perpetually flighty where her kids are preternaturally well-behaved, Keiko (played by a former singer and television personality whose name is “YOU”) seems at first to be a perfectly loving mother, determined to hold the family together even though each of her children was fathered by a different man.

But Keiko’s desperate need for fulfillment as a woman soon seeps through the cracks in her sunny maternal side. One day she leaves some money on the kitchen table along with a note asking Akira to take care of things for a few days; she eventually returns, but only long enough to pack some clothes and head out the door, this time with appalling finality. Akira proves as resourceful a surrogate parent as any insufficiently educated preteen might. But it’s only a matter of time before fate comes calling and the contents of the ominous red suitcase are finally revealed.

But Nobody Knows is, first and foremost, a film about these four kids. Every shot reveals the intimate trust that developed between the actors and their director, who worked together in a single cramped apartment over the course of an entire year. In the claustrophobic, crayon-drawing chaos their lives become, and in every close-up of their grubby fingers and stocking-covered feet, Kore-eda makes giants of these prematurely independent beings even as he refuses to let us repeat the sins of their parents. Peeking into a world where four unforgettably fragile throwaways are forced to grow up all too quickly, Nobody Knows never stops reminding us that bigger doesn’t always mean better.

NOBODY KNOWS ( * * * ½ )
Directed by Hirokazu Kore-eda.
Starring Yuya Yagira, Ayu Kitaura and You.
(Rated PG-13, 141 mins.) At the Osio Cinemas.