Eden Lake (18)

At last! Here's a first-rate British horror film that taps into our deepest fears and offers a thought-provoking insight into such topical subjects as knife crime and gang culture.

Though nightmarish and visceral, it's the most intelligent horror film to have been made by a British director since Jack Clayton's The Innocents in 1960. And it fulfils the two purposes of horror: it involves you emotionally and it's frightening.

One of our finest young actresses, Kelly Reilly, plays Jenny, a primary teacher first seen playing peek-a-boo with her charmingly innocent pupils. She's as pretty, defiantly English and child-friendly as Julie Andrews in The Sound Of Music.

Kelly Reilly is transformed by violence in a fantastic performance

She goes off camping for the weekend with her boyfriend Steve (Michael Fassbender), little knowing he is about to propose marriage.

Ominous portents abound: a woman slaps her child in a supermarket; rude cyclists casually ride through a red light, causing Steve's Jeep to brake; and an advertising hoarding announces that Steve's favourite lake is to be redeveloped into a gated community.

'Who are they so afraid of?' asks Jenny, with the blitheness of someone probably recruited for her job through the pages of the Guardian. She's about to find out.

Jenny and Steve laugh as they leave the road and receive a menacing message from the bossy voice on their sat nav: 'At your first opportunity, turn around.'

They swim, sunbathe and enjoy the peace of the lake, which is a man-made quarry. But their idyll is spoiled by teenagers playing their music loud and failing to control their Rottweiler.

Jenny wants to move on. 'Boys will be boys,' she says, not noticing that one of the gang is a girl.

Steve is all for approaching the group and believes they'll see reason. Jenny says: 'No, Steve, it's not worth it.' But he replies: 'If everyone said that, where would we be?'

It's a thoroughly credible set-up and the process of escalation whereby Jenny and Steve alienate, then anger these feral youths until they're ready to stab, torture and even burn them to death is worryingly authentic.

Unlike most horror films, in which the heroes steer themselves into danger by their own stupidity, Jenny and Steve behave with complete plausibility and a tragically unrequited sense of kindness and social responsibility.

Be warned that events turn extremely gruesome. Even though I have been desensitised by many hours of watching bigscreen violence, I found some of it hard to watch. But for once the violence has a point, and it's treated responsibly.

Never has knife crime been less glamorised. We watch someone bleeding to death from stab wounds, and it's a shocking sight. And there's darkly comic horror to be experienced as we watch the ringleader's girlfriend recording acts of extreme violence impassively on her mobile phone. We see the horrifying effects on Jenny as she's transformed from a nice young teacher into - well, I'll leave you to find out.

Unlike the torture porn it superficially resembles, it makes sure our sympathies are always with the victims.

Eden Lake delivers plenty of tension and vicarious excitement. But it's also willing to say what other films have been too scared or politically correct to mention: the true horrors we fear day to day are not supernatural bogeymen or monsters created by scientists. They're our own youth.

This film will doubtless be accused of class hatred and demonising chavs, especially by those who accuse newspapers such as the Daily Mail of whipping up public concern over innocent victims of street gangs.

The obvious point never seems to occur to these people that we are right to feel concerned about the stabbing of headmaster Philip Lawrence outside his own school gates in 1995 or the way the murderers of Stephen Lawrence are walking free. We wouldn't be fully human if we weren't. And it's made abundantly clear within the film that the guilty youths are often the offspring of parents who have jobs and might be called middle class, but who have lost their moral compass and any feelings of responsibility towards their children.

Far from demonising the gangmembers, there's an underlying compassion towards them, along with a sad realism about mankind's potential for barbarism. It's the same feeling that made a classic of William Golding's Lord Of The Flies.

Kelly Reilly finds herself in a predicament in Eden Lake

The film is remarkably strong on the dynamics of the gang, with its psychotic leader (played by Jack O'Connell as a variation on Malcolm McDowell in A Clockwork Orange) bullying, manipulating and incriminating his less daring colleagues until they become as vicious as he is.

You can see from his attachment to his dog that she is the only love he has in his life - and wait until you meet his parents.

Uncaring or absentee parents are the true villains, mostly unseen until the unpleasant end, but an implied presence throughout.

There's a world of pathos in the reply of a boy whom Jenny meets in the woods and asks: 'Where's your mother?' (obviously considering it's safest to assume he has no father).

He answers sadly: 'She's working.'

Time will tell if first-time writer-director James Watkins is a major new talent or just a flash in the pan. Either way, he has made a terrific directorial debut.

Even though the picture was shot in six weeks on a limited budget, I was taken with cinematographer Chris Ross's use of backlighting at the start, when the couple seems to be embarking on an idyllic holiday, and his shift to harsher, less flattering light and hand-held camerawork as things go wrong.

Eden Lake may sound like yet another story about nice people straying into hostile territory - like Straw Dogs, Deliverance or Timber Falls - but it has five virtues you don't often see in horror films: believability, leading characters you care about, a responsible attitude towards violence, a willingness to get inside the heads of the 'bad guys', and a recognition that aggression may brutalise even the victim.

A sixth virtue is that it doesn't fight shy of a truly frightening final twist, which makes it not only bleaker but also more truthful than virtually every other movie in this genre, which all too often is over-populated and under-humanised.

Verdict: An excellent British horror film