When, in the early stages of this harsh, nihilistic depiction of life in contemporary Mexico, a young girl happens upon a cute little puppy, informed viewers will brace themselves for impending canicide. Sure enough, within half an hour, gun-wielding vandals have broken the unfortunate beast's neck.

That's what happens in harsh, nihilistic depictions of life in contemporary Mexico. A few moments later, a young man has his penis set on fire. At this point, when Heli screened at Cannes last year, more than a few timid reviewers placed hands over mouths and tiptoed for the exit.

It's worth being facetious about such issues, because, otherwise hugely powerful, Heli lets itself down at these points. Amat Escalante, director of the equally troubling Sangre, has made an impressively sinister drama about the way (to paraphrase Philip Larkin) man hands misery onto man. Unfortunately, incidents such as the one with the doomed puppy seem more than a little gratuitous. It's as if Escalante is cutting his own skin so that he may later ostentatiously pick at the scab.

The excellent Armando Espitia plays the titular car worker. As the film begins, Heli is living a humble but tolerable sort of life. Then Estela (Andrea Vergara), his young sister, becomes involved with an older police cadet named Beto (Juan Eduardo Palacios).

When the lad steals a bag of cocaine, he ends up attracting the hideous wrath of a gang of crooked cops. They break into Heli’s house, kill his father, torture him and Beto, strangle the dog and kidnap Estela.

Shot with a cool distance, Heli has none of the gooey relish of a Gaspar Noé film. This is a less connected cinema of cruelty. But the gradual slump towards ultimate moral compromise – played out to an apparent quote from John Ford's The Searchers – is no less gripping for all that. The violence may be gratuitous, but the anger at social inequality feels entirely sincere. Powerful stuff.