Great films change the way we think about cinema. A masterpiece can alter our perception of life. Matteo Garrone’s startling film, Gomorrah, about the criminal underworld in Naples, is one of these rare movies. The title is a biblical pun on the Comor-ra gangs who run the city’s sink estates like private businesses. It’s a gripping fiction about the appalling damage they cause. The opening sequence is a sardonic nod to the classic Mafia murder. A group of tattooed men are topping up their tans in a grubby solarium. The screen is a toxic glow of ultraviolet.

The Friday banter comes to a grisly halt when a smiling thug fishes a gun from behind his back and shoots his half-naked friends at point-blank range. We are not privy to the reasons for this. The perplexing lack of narrative turns every unpredictable killing into a squalid waste of life.

Garrone’s film is brave to the point of foolhardy. Gomorrah is a lacerating account of the real Camorra, the mindless violence, and the slums where they breed. Garrone delivers a sledgehammer blow to the Godfather fantasy. He shatters the Hollywood myth and exposes the godforsaken reality. The only “tradition” celebrated here is callous indifference.

The documentary intensity of Gomorrah is a tabloid scandal. The crumbling estates of Scam-pia are home to families whose lives are determined by overweight skinheads in cheap track-suits. The crowded flats are incongruously stacked up in the shape of ocean liners beached next to eight-lane highways in the middle of baking nowhere. The concrete gangplanks are patrolled by seedy drug-dealers.

The film stalks a handful of vulnerable locals. Totò is an androgynous 13-year-old who is desperate to sign up to a gang, even if the initiation ceremony involves being shot in the chest. Two pimply Scarface fans, Marco and Ciro, steal guns from the wrong crew. And a sleazy flake, Franco, mints a fortune by tucking away lorry-loads of chemicals in local quarries.

The brilliant cast are a terrifying mix of professional actors and hand-picked locals. The unpredictable narrative is far more in tune with the favelas in City of God than Scorsese’s Mean Streets. In some respects Gomorrah marks a parting of the ways. This bleak portrait of human waste has little chance of paddling across the Atlantic. Roberto Saviano, the young Italian journalist who wrote the book on which the film is based, has paid a high price for this authenticity. He is now living with police protection.

15, 137 minutes

Editor's choice

• The 100 Best Films of the Decade
• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010