The government of Kazakhstan has railed against the spoof documentary Borat, which portrays the country as a medieval society in which rape and incest are cheerfully accepted, and sometimes combined. Audiences will spot from a mile off that the joke is not on the country itself, but on the western perception that foreign cultures are backward. The comic Sacha Baron Cohen uses the character of Borat Sagdiyev, Kazakhstan's second-best reporter, to reveal the true nature of the Americans he meets. Surely the Kazakh government has better things to do than take umbrage at such a lucid campaign against xenophobia. Like discovering fire, for instance. Or stopping all the menfolk from sleeping with their own sisters.

Visiting the US on a fact-finding mission with his producer, Azamat Bagatov (Ken Davitian), Borat stops first in New York, where he is tutored by a humour coach in American wit, which consists of adding the exclamation "Not!" to the end of every sentence.

It soon becomes apparent that the film's primary function will be to bring Borat into contact with those he is most likely to irritate. A meeting with the Veteran Feminists of America gets off to a bad start when Borat fails to contain his amusement at the idea that women are equal, and advises one of the group ("pussycat") to "smile a bit". It's nothing more sophisticated than a prank at the expense of those who don't know what the joke is, or even that there is a joke. We get a kick out of Borat behaving badly with impunity and saying the unsayable. There are, however, some passages - such as his misadventures in an antiques shop - in which only Baron Cohen's devilish performance allays suspicions that we are witnessing the second coming of the TV prankster Jeremy Beadle.

The picture bares its claws when Borat and Azamat cross the US in an ice-cream van in search of Pamela Anderson, with whom Borat has become infatuated. The focus of the comedy shifts as Baron Cohen quizzes assorted Americans and gives them enough rope to hang themselves several times over and still have enough left to tie a big floppy bow on top.

There's an ageing cowpoke who requires only the mildest of prompts to endorse the murder of gays and Muslims. Others indict themselves as much by what they don't say as what they do. A redneck rodeo crowd shows no compunction about cheering Borat's gung-ho speech about Iraq, clearly not realising that what he actually said was: "We support your war of terror!" And it's shocking to witness the tacit acceptance with which Borat's ghoulish requests are greeted. Trying to find the ideal car for mowing down gypsies, or seeking the best gun for killing Jews, he encounters only compliance among America's salespeople. The customer, it seems, is always right, even when he's far right.

Borat is directed briskly by Larry Charles, the Curb Your Enthusiasm stalwart. If there is an impediment to the film's effectiveness, it lies in the clash between scripted material and vérité footage. The violence that Borat encounters on the New York subway after trying to greet male strangers with kisses is frighteningly real. On the other hand, we know that the prostitute who joins him at the Magnolia Mansion Dining Society is an actress, and can giggle along accordingly.

But there is a grey area in which encounters presented as documentary have a bogus ring, such as Borat's journey with three rowdy frat-boys who seem suspiciously on-message. The staged gags are inevitably eclipsed by scenes that carry a whiff of authentic danger - Baron Cohen is such a kamikaze performer that he whips up a taste not just for the tears of the clown, but for his bruises and broken bones, too.

Pick of the week

Red Road (18)
dir: Andrea Arnold
Glasgow is the backdrop to one of the best films of the year.

I Saw Ben Barka Get Killed (12A)
dir: Serge Le Péron
Conspiracy theories abound in an intriguing political thriller.

Little Children (15)
dir: Todd Field
Kate Winslet and Patrick Wilson play adulterous parents in a drama from the maker of In the Bedroom.