If you wrote a capsule summary of Sin Nombre, a riveting melodrama set in Honduras, Mexico, and (ever so briefly) Texas, it would be easy to make the movie sound like an earnest snooze. “In a film made without professional actors and shot entirely in Spanish, Central American migrants experience hardship and solidarity in their long quest to reach the United States” would do the trick, I think — especially if you threw in the film’s backstory as the thesis project of a precocious half-Japanese, half-Swedish northern Californian named Cary Joji Fukunaga, and noted that it earned raves and prizes from the socially conscious jet-setters at last year’s Sundance Film Festival.

But if you ignore the movie’s provenance and just translate the title into English, you’ll have a better sense of what Fukunaga’s movie is all about. Sin Nombre means “Without a Name,” three words that conjure, tersely, the archetypes of the American West, and the long shadow of Clint Eastwood and Sergio Leone. And the movie vindicates these allusions. The gritty social realism that its subject matter promises is interwoven with a bloody, Biblical narrative about honor and brotherhood, love and murder, in which cycAles of revenge and redemption churn like the wheels of the train that carries the migrants toward the Rio Grande.

Sin Nombre starts slowly, in the Honduran capital, Tegucigalpa, and the dirt-poor Mexican state of Chiapas, taking its time introducing the two families whose unexpected intersection drives the plot. The Honduran family belongs to the teenage Sayra (Paulina Gaitan), whose widowed father (Gerardo Taracena), gone from her life for years, has just been deported home from the distant promised land of New Jersey, where he’d acquired a new wife and several children. Their reunion is awkward, but not awkward enough to dissuade Sayra from accompanying him and her uncle Orlando (Guillermo Villegas) when they set out for the United States once more, slipping across the borders into Guatemala and then into Mexico, where a trainyard in Chiapas provides a jumping-off point for the host of migrants heading north.

It’s also where their paths cross with the story’s second family, which is tighter-knit and considerably more terrible. This is the Mara Salvatrucha gang, known to many Americans as MS-13, which rules the barrios from Los Angeles — where it got its start in the 1980s, among Central Americans far from home — all the way to Nicaragua. In Chiapas, it’s enfolded the brooding Willy (Edgar Flores), who serves at the whims of a tattooed, bright-smiling boss named Lil Mago (Tenoch Huerta) and mentors a pre-teen would-be gangster named Smiley (Kristian Ferrer).

Enfolded, and entrapped. Willy, dubbed “El Caspar” by his brothers in the gang, has moral qualms about the brotherhood he’s entered. You can see his alienation when its requirements are demonstrated, early on, by the violent hazing of the still-innocent Smiley — and then, in one of the movie’s few lurches into the cinema of exploitation, by the execution of an unarmed rival, and the subsequent appearance of his entrails in the slop buckets that feed the Mara’s dogs. What’s more, he’s fallen into the gravitational pull of a middle-class girlfriend, Martha (Diana Garcia), whom he visits on the sly, climbing into her pink-curtained bower for assignations, making love amid intimations of a different, better life.

In the heart of a discontented gang member, stirrings of conscience and a luminous bourgeois girlfriend make for a dangerous combination — dangerous enough, in fact, to create the sequence of unexpected events that throws most of the characters (most of the surviving characters, I should say) together on a train car, careening their way through the Mexican countryside, with various Mara Salvatrucha foot soldiers hot on their heels.

The twisty, jackknifing story that follows lends itself to plot summary only if you like to know going in who lives and who dies, so I’ll leave off summarizing there, and just promise that while you’ll probably see the final sequence coming, you’ll be reliably surprised — and reliably riveted — by everything that happens up till then.

What’s more, you’ll be swept up in it. Sin Nombre has a shoestring budget but an epic feel. The camera follows the train, and rides it, devouring Mexico as it moves: mountains and rivers, vine-clotted buildings and crimson-dappled fields. Children run alongside, throwing fruit up to the migrants; later, a different group of children run alongside throwing stones. The heavens open, and the crowds on the train tops vanish under colored tarps and ponchos, like a field of flowers blooming under rain.

The performances are naïve, archetypal. The actors and their characters are adolescent, angst-ridden but unencumbered by deep complication. Their motives are primal and unsubtle: These are Romeos and Juliets, not Hamlets. They crave love, purpose, adventure, approval — and revenge. They get what they want; they don’t always like it. You will.