“Nobody Knows,” a profoundly moving film from the acclaimed Japanese filmmaker Hirokazu Koreeda, proves that the greatest movies don’t need flashy action or CGI effects to achieve significant emotional impact. Despite its immediate appearance of stripped-down starkness, it is a film of technical virtuosity and carefully orchestrated sequences. But the true heart of the film — which is loosely based on the true story of four children abandoned by their mother in Japan in 1988 and discovered six months later — lies in its shocking realism.

“Nobody Knows” begins when Keiko (played by the Japanese pop star You) deserts her young children in a run-down apartment in a nameless Japanese city with barely enough money to pay the bills. Her oldest son Akira (Yuya Yagira) must fend for himself and protect his younger brother Shigeru (Hiei Kimura) and his sisters Kyoko (Ayu Kitaura) and Yuki (Momoko Shimizu). Akira tries his best to be a parent, borrowing money from dishonest family acquaintances, buying Christmas gifts for his siblings and relying on new friends for help, including the young Saki (Hanae Kan).

Many films have captured the gritty experience of urban survival in a busy and unfriendly city, and plenty are told from the perspective of children. But unlike movies such as the recent “In America,” this story is characterized by an utter lack of sentimentality and an extraordinary subtlety. The movie merely hints at the family’s past before the opening of the film — Koreeda is wisely content to develop his characters through action without succumbing to unnecessary narration or expository dialogue.

As such, the storyline of “Nobody Knows” is a loose framework rather than an intricate plot. With sparse dialogue and minimalist production, the film feels wholly authentic, even documentary-like (not coincidently). Koreeda actually hired unprofessional actors, working with them in free-form improvisation and filming the story chronologically. As a result, the performances are astonishingly convincing — the actors literally age on-screen. (Yuya Yagira, a novice, won Best Actor at the Cannes Film Festival.)

Koreeda is keenly aware of the power that a slowly unfolding story can have. The film’s quietness and slow pacing make the few plot developments and revelations all the more emotional. Rather than relying on dialogue, Koreeda reveals characters’ emotions and thoughts in ways that many directors never attempt, through lingering smiles and exchanged glances. The intense connection to these characters and the ultimate emotional payoff at the climax are results of these stylistic choices.

Above all, Koreeda is a brilliant visual storyteller. With his cinematographer Yamazaki Yutaka, he creates a style that perfectly complements the fly-on-the-wall nature of his movie. The camera is patiently kept in one place, and as result much of the action takes place on the edges, even outside, of the frame. Although Koreeda and Yutaka carefully plan each shot, the film’s cinematography still feels experimental and beautifully authentic. Yutaka’s garish lighting and the set’s close quarters, emphasize the seclusion and loneliness of the children’s apartment. This symbolism extends to the repeated isolation of visual patterns (Akira on staircases, for example) to demonstrate the hopelessness of the children’s existence: As the children revisit old places, each time things seem worse than before.

But, as he did with 1998’s “After Life,” Koreeda finds unexpected humor and optimism even in the darkest of situations. At one point, Yuki’s hilariously squeaky shoes represent the joyous privilege of leaving the house for the first time. In another scene, hand-held camera movement and rapid editing are used to show Akira’s exhilaration as he watches a train pass quickly by, dreaming that one day it will take him far away.

Too many contemporary Hollywood films are content to tell rather than show. Koreeda’s authentic direction is a refreshing reminder of cinema at its quiet best. But more importantly — and perhaps more surprisingly, given the simplicity of its style and its plot — “Nobody Knows” is highly affecting and entirely engrossing. It also constitutes a major social statement: In the late 1980s, when the plight of the four abandoned children came to light, many Japanese were shocked that their society had ever allowed this to happen. But the film, with its patient camera work and natural storyline, make their dark fates seem all too familiar.