Without a moment of cheap sentimentality, "Lean on Pete," a story about the tragedies and resilience of youth, holds the torch for emotion pretty high. If "The 400 Blows" was set in the poorer sections of the rural Pacific Northwest, it would run very close to this.

Director Andrew Haigh, having shattered hearts in a film about a one-night stand ("Weekend") and another about loss of love after decades ("45 Years"), here examines neglectful adults and children's needs to find refuges away from home and inside their heads. It's fundamentally a love story between 15-year-old lost boy Charley (Charlie Plummer) and a chestnut-colored colt named Lean on Pete.

Charley's life hasn't offered him much of a safety net. His mother abandoned him long ago. His father disarmingly jokes with Charley like an older brother, but his single-parenting skills are almost nonexistent, his income as a driver is near the poverty line and he tends to vanish unreliably when a woman of interest crosses his view. The only item Charley can be sure to find in their refrigerator is the light bulb.

When Charley stumbles into a part-time job as a groom for small-time racehorse owner Del (Steve Buscemi), he not only gets a little cash to pocket, but he acquires a worldly wise mentor, as well.

In short order, Charley's father accepts the new arrangement, and Del takes the boy to nickel-and-dime racetracks to water, brush and walk his horses. From his new insider's perspective, Charley learns some unpleasant facts about the races. Del regards his horses as investments, doping them for the best performance they can deliver without being disqualified by veterinarians and selling them at the end of their galloping days to Mexican buyers. He's not saying whether they're sold as livestock or dog food.

For the time being, Pete's rider is Bonnie (Chloë Sevigny), a friendly, skeptical jockey on the verge of giving it up after her own injuries. "There are only so many times you can fall off a horse and get up," she says. For her, too. Pete is not a pet, only a horse. But as the boy and the animal bond, Pete becomes Charley's only reliable friend.

The film offers sort of a low-budget travelogue across the American West. Through their arduous journey together, Haigh treats their challenges with complete understatement and objectivity that, while not unfeeling, does not parade its emotional state.

There is no specifying, moralizing or commentary of any sort, just hard difficulties observed with acute accuracy. It all looks, feels, and smells as real as the cheap diners where Del impatiently tries to teach Charley the proper way to eat in public. Time and again, we're gently reminded that the boy is not simple, but has a lot to learn.

There's a talented ensemble at work here. Plummer has a gift for radiating naiveté that fits like a jigsaw piece against Buscemi's weary seen-it-all cynicism.

Plummer, last seen as J. Paul Getty's dissolute, druggie grandson in "All the Money in the World," has a challenging part as Charley, a complicated adolescent presented in spare dialogue and unsparing images. He is impeccable, even in sequences where he must handle horses, which are noble creatures but not the most reliable actors. The slow step by step of Charley's awakening to life's casual cruelty is done without a hint of performing, which is the highest compliment such work can earn.

As a new acquaintance who becomes Charley's tutor in street life, Steve Zahn is equally good and quite unpredictable. Everyone here supplies a human face, even to characters who are not necessarily humanizing.

"Lean on Pete" is a film in which the acting is almost invisible, but the Dickensian message about the pain and joy of growing up can be appreciated by anyone who has felt unwanted, unloved or uncared for. It's a film, in other words, for almost everyone.