There are feel-good films and feel-bad films. And then there are feel-downright-uncomfortable films that leave you squirming and gasping for two hours. The British thriller Red Road belongs in the latter category: it's a wonderful surprise that's full of nasty shocks.

Jackie (Kate Dickie) is a CCTV operator monitoring the cameras that overlook the mean streets of Glasgow. Her closest cinematic precursor is James Stewart, poking his long lens into his neighbours' business in Rear Window (1954). Where Stewart had the luminous Grace Kelly to lighten his mood, Jackie's leisure pursuits include having miserable sex with a married man in a van while his dog takes a dump outside. Her companion's idea of foreplay is to turn off the engine. It might not sound romantic, but at least Jackie gets a lift home afterwards.

One night during her shift, she witnesses what she believes is a rape about to happen, and manoeuvres the cameras to track the potential assailant. When it turns out to be nothing more than a consensual quickie, she is relieved. Then she recognises the man on the screen. Jackie's mortified expression rules out the possibility that he is an old school chum with whom she's been meaning to get in touch.

This is Clyde (Tony Curran), fresh out of prison and acting like sex on legs. What has Clyde done to Jackie? And what does she want to do to him? In both cases, the answer is: something horrible. She begins by stalking him, and the subjective camera-work leaves us no choice but to stalk him, too, which is highly disconcerting. Andrea Arnold, who writes and directs this as her first feature, clearly intends to specialise in taking us to places we'd rather not go and introducing us to people we'd rather not meet. What marks her out as a distinctive film-maker is her ability to make us care about every character, no matter how unsavoury they might seem.

One watches most of Red Road in a state of dread, though it's not a humourless film. I loved the Daily Record headlines that the camera keeps glimpsing, such as "Dumbarton man in black pudding scare" or the card in a newsagent's window that reads: "Magician Available For Parties - Can Saw In Half". It's a gentle reminder that Scotland is the land not only of Trainspotting and Sweet Sixteen, but of Bill Forsyth.

Glasgow is portrayed at first as positively rancid; if the picture were shot in Odorama, the prevailing smells would be vomit and chips. But Arnold's refusal to keep playing the grittiness card does her film proud. Instead, the city becomes an eerie netherworld with hints of Ovid or Jean Cocteau. Feral dogs outnumber people, and human beings are easily mistaken for beasts. "You're an animal!" a waitress giggles as Clyde's hand climbs her thigh. After Jackie first sees Clyde on CCTV, he vanishes into the long grass on a stretch of waste ground. From the other side of the undergrowth emerges a lean fox that slinks across the highway into the night. Of course, it's an editing trick: Clyde hasn't actually turned into an animal. But when, in a later scene, he admires the brutal cries of foxes mating, you feel a chill down your spine: he is like Bela Lugosi as Dracula, cocking an ear to the howling wolves and proclaiming, "Children of the night - what music they make!"

On the evidence of this debut, there is every chance that Arnold will make a masterpiece, though Red Road isn't quite it. In the last 20 minutes of the picture, the complexities that have enthralled us are waved away as if with a magic wand. But you can forgive her this wrong turn, because every shot, every cut, until then has been alive with imagination and daring.

Red Road is the first in an intriguing project, co-produced by Lars von Trier's Zentropa company, with Arnold one of three directors assigned to make a film set in Glasgow. The same characters, played by the same actors, will appear in each picture, so we can expect to see Jackie and Clyde again in different circumstances. For the sake of our anxiety levels, here's hoping their next outing is a slapstick comedy.

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