Man cannot subsist on glamour alone, and Cannes knows it. So, after the sugar rush of opening nighter The Great Gatsby, the programmers scheduled in some veg. It was served New Wave Mexican style: raw, gritty, and force fed by bandits who snap puppies' necks with one hand while recruiting underage sex slaves with the other. It tasted as superficially indigestible, if ultimately nutritious, as the prickly pears our hero hacks off the desert cacti in a frenzy of impotent rage.

Heli (Armando Espitia) is about 20, and lives with his wife, baby, father and 12-year-old sister Estela (Andrea Vergara). This we learn when a census officer pops by his breeze-block house – a half-cute, half-clumsy device – just before he hops on his boneshaker for the night shift at the local auto factory. He forgets himself when he tots up the numbers; this is structurally, as well as thematically, a film about supporting others as you yourself are written out of the picture.

His sister is seeing a 17-year-old soldier, Beto (Juan Eduardo Palacio); scenes of front-seat fumblings seem initially queasy, then, with hindsight, the picture of innocence and joy. For Beto, whose training includes rolling about in his own vomit, has stolen some cocaine and stashed it in the water tank on the top of Heli's house. This triggers a fallout which eventually explains the opening shots (a kidnapped man being hung from a footbridge), by way of an extended torture set-piece involving cricket-bat beatings and a set of genitals doused in lighter fluid and flambéed, Christmas-pud style. A sofa-full of bored boys watch, taking turns with the bat as a break from the Wii. "What did this one do?" asks one. "Who knows," says another. In the kitchen next door, an unconcerned woman makes tea, cutlery clattering in the gaps between the screams.

Catherine Shoard interviews Heli director Amat Escalante guardian.co.uk

Will justice be done? Will it heck. Even the most apparently sympathetic lawmaker is out for something from Heli. As the third act progresses, creeping paranoia seeps into even the most innocuous moment, so the film functions as clammy thriller as well as poetic agitprop. Director Amat Escalante – a more explicitly political blood brother to Mexico's current king of neo-realist weirdness, Carlos Reygadas – has shot a damning indictment of contemporary Mexico, capturing its institutionalised corruption, its endemic cruelty.

What makes it palatable are the grace notes: a cowboy with the curliest boots in the world, a Ray Cooney-ish reveal from a lady cop, a scene in which Beto proves his machismo to Estela by bench-pressing her. Squint, and the title makes more sense. Shut your eyes entirely to its horrors and you'll really miss out.