Saddest movie I doubt many of you have seen...

The horror of "Nobody Knows" descended upon me later, when the film was over and I was considering its effects. It is the very definition of "haunting," in that it has a strong impact in the viewing, but an even stronger one in the contemplating.
Loosely based on a true story, it begins with Keiko Fukushima (one-named actress You) moving into a small apartment with her 12-year-old son Akira (Yuya Yagira). She checks in with the husband-and-wife landlord team, exchanges pleasantries, and assures them Akira is her only child, so they needn't worry about noise or clutter. The landlords are happy.
Then the Fukushimas unpack their suitcases and reveal two stowaways: Keiko's other children, 5-year-old Yuki (Momoko Shimizu) and Shigeru (Hiei Kimura), about the same age. A fourth child, a girl named Kyoko (Ayu Kitaura) around 10, arrives by train later that night and is secreted into the apartment where the family of five will now live, three of them hiding, Anne Frank-style, from the world.
Mom is a sad case. She maintains a playful, easy relationship with the children, reminding Yuki and especially the energetic Shigeru of the importance of staying quiet and never leaving the apartment. They do not seem unhappy by the restrictions; probably this is life as they've always known it, bouncing from one apartment to another as Mom bounces from one low-level job to another. We gather that each of the children has a different father -- Akira is on a first-name basis with a few of them, who work in various menial jobs around town -- and that Keiko has never been married. She is flighty and silly, coming home very late one night, drunk, and waking up the kids so they can eat sushi with her. She thinks it sounds fun; the kids just want to sleep.
Keiko begins to be away for long periods of time, always leaving enough cash behind for Akira to run the household and buy groceries. Her absences grow longer and longer, her stays at home shorter and shorter, and finally she stops coming home altogether. She has abandoned her children.
Akira and Kyoko become the family's "parents," washing the dinner dishes while they discuss Shigeru's reluctance to eat his vegetables, Akira doing the shopping and errands while Kyoko tends to the younger children at home. None of them go to school, and they can't tell the police of their plight for fear of being split up into different foster homes. Besides, the situation doesn't strike them as dire. Mom left some money and continues to send cash now and then from her new home, wherever that is. Akira has grown accustomed to being the adult in the family; all that Mom's disappearance has done is make it official.
But you can only do something unnatural for so long before nature rebels. Akira is a young boy on the bring of adolescence. He goes through a phase where he befriends some trouble-making local boys simply because he wants the normalcy of childhood. He invites them over to play video games, pushing his siblings out of the way, shirking his family duties simply by being what he's supposed to be: a kid. In one heartbreaking scene, he takes Yuki into the city because it's her birthday, the little girl realizing Mom probably isn't coming back but so used to being neglected that now it just rolls off her. In another scene, Akira finds a baseball in the park and hits it with a stick, that simple childlike pastime bringing him unimaginable joy. And then it is back home to the kids.
The money runs out eventually. The electricity and water are shut off. The children make friends with a local girl named Saki (Hane Kan), a sad waif who sympathizes with their loneliness, though they withhold the complete details of their situation from her, and it looks like she will be of some help to them. But they don't need another minor looking out for them. They need parents.
Japanese filmmaker Hirokazu Koreeda ("After Life") shot "Nobody Knows" in chronological order and took a year to do it, which means the children age, almost imperceptibly, over the course of the film. More noticeable is the gradual descent of the little family from neatness to squalor as they slowly lose sight of the real world, though even there, Koreeda avoids melodrama. I didn't even notice until afterward that at some point, the children stop keeping the secret their mother gave them (that there are four of them in the apartment, rather than one) and start going outside boldly, probably because they have realized that the world doesn't care about them anyway.
The film ends with a sense of hope, but also with a sense that the hope is false. The young actors give such disciplined, believable performances as to be heart-wrenching -- not because they overdo the emotional aspects or play to our sympathies, but precisely because they DON'T do that. The children remain so stoic, their determination barely masking their youthful innocence, that we can't bear to think of their story having an unhappy ending.