Eden Lake (18)

The horrors lying in wait for good-looking young couples on holiday are now so well known from the movies that "taking a break" is practically shorthand for inviting disaster.

America has led the way in this gruelling little sub-genre, which in the last year alone has put us through Vacancy, Timber Falls, the remade Funny Games and The Strangers. In Blighty, we don't have hillbillies with rifles and steel-traps, but we do have hoodies with knives and mobile phones, and they prove quite frightening enough in James Watkins's grippingly unpleasant debut, Eden Lake.

The film starts out with an implicit question. How far into the country do you have to go to escape the hell of other people – or, rather, other people's children? A couple on a weekend break seem to have found an Elysian wilderness in the woods by Lake Eden: as Steve (Michael Fassbender) tells his girlfriend Jenny (Kelly Reilly), the place is about to be invaded by a development of 50 "executive homes". But in the meantime their cosy spot is invaded by a gang of teenage kids, who first give the couple a lesson in charmlessness – loud music, a slavering dog, an air of loutish menace – and, later, a lesson in ruthlessness.

How bad does it get? Put it this way; Jenny and Steve will soon need tetanus jabs. Watkins, who co-scripted one of the decade's best thrillers in My Little Eye, builds the mood of threat nicely, starting in the B&B where the couple stay overnight and then tightening the noose around them once in the woods.

Jenny, a primary-school teacher, has a natural affinity with children, but she has probably never met one like Brett (Jack O'Connell), a potato-faced youth of astonishing malice. His mates, the script implies, are not naturally "feral", but become so under his constant intimidation. It's not enough any more to happy-slap strangers; now you have to prove your hardnut credibility with a knife.

The stalk-and-chase scenes through the woods are nothing new, though Watkins gives us an occasional jolt with an aerial shot of the couple fleeing blindly through this unfriendly terrain. I felt especially sorry for Reilly, who endures dreadful indignities in the name of realism and ends up slathered in primordial slime. Not a good look.

While Eden Lake poses uncomfortable, possibly unanswerable, questions about gang culture and the disappearance of respect, one detects a more ambiguous conflict being raised. It's not just about the adult fear of children, it's about the middle-class fear of a violent underclass – the sort who start off clouting their children and then renounce any responsibility for their discipline. Having prompted us to wonder where the parents of these terrorising minors are, the film pulls its final trick by driving us right to their front door. What happens then isn't entirely credible, but it is, in storytelling terms, quite convincing. We have seen a couple's idea of personal security – car alarm, mobile, satnav – comprehensively trashed; their assumptions of common decency and fair treatment follow quickly after. Daily Mail scaremongering? Possibly. But formidably well-made, all the same.