After Aliens3, Seven and The Game, you do not go to a David Fincher film expecting realism. His films are dark, apocalyptic fables set in imaginary cities, paranoid tales that jangle millennial nerves, and Fight Club is the most jarring thing he's done because it takes us inside the skull of a disturbed man. Both the movie's male stars - Ed Norton and Brad Pitt - have specialised in playing lunatics, fascists, psychotic killers, and together they're combustible.

Based quite closely on a novel by Chuck Palahniuk, Fight Club belongs to a tradition of fiction that stretches back to the Bible, in which the protagonist confronts a beguiling stranger that he seems to have conjured up. In the earlier versions this figure is clearly Satan or his representative.

But since the romantic movement, the doppelgängers in books, ranging from James Hogg's Confessions of a Justified Sinner of 1824 to Martin Amis's Money of 1984, by way of Poe, Dickens and Dostoevsky, are more problematic. The nineteenth-century examples anticipate Freud's writing on the ego, superego and id, the twentieth-century ones are steeped in Freud, and the whole subject is comprehensively dealt with by Karl Miller in his brilliant book, Doubles.

The unnamed Narrator of Fight Club (Ed Norton) is first seen at night, battered and bleeding in an unfurnished room of a glass skyscraper, a gun stuck in his mouth. In flashback, he explains in voice-over how he came to be there. He was a 30-year-old yuppie, living alone in an immaculate flat in an anonymous apartment block and devoted to consumerism. Discontented with the good life and his well-paid, somewhat dubious job as an accident calculator for an automobile firm, he's troubled with insomnia.

A doctor tells this unhappy loner that if he wants to see real suffering he should visit a support group for men suffering from testicular cancer. In a darkly comic fashion, he becomes addicted to support groups for the terminally ill, going to a different one every night. 'When people think you're dying they really listen to you,' he remarks.

Along the way, he meets another 'disease tourist', the death-seeking Marla Singer (Helena Bonham Carter), a whey-faced skid-row Liz Taylor, and they agree to divide up the support sessions so they don't meet. This is brilliantly funny in a manner that recalls Crash, but it's shot out of sequence in a fashion gauged to puzzle and disorient.

Then, on one of his frequent plane trips, the Narrator meets the raffish, charismatic Tyler Durden (Brad Pitt) who transforms his life. One recalls Raymond Chandler's working note for his screenplay of Strangers on a Train : 'If you shake hands with a maniac you may have sold your soul to the devil.' Tyler is a silver-tongued prankster, the bad boy your mother warned you to keep away from, the id in its most uncontrolled form.

The Narrator moves into Tyler's spectacularly squalid home, a rotting mansion on an urban wasteland that might have belonged to some poor relatives of the Addams family. Tyler is the enemy of bourgeois conformity, a moral saboteur on the lines of Guy Grand, the malevolent hoaxer in Terry Southern's Magic Christian. He works as a waiter to pollute the food of smart restaurants, uses his job as a projectionist to cut subliminal pornographic frames into Disney pictures (one of Guy Grand's tricks), recycles the fat extracted from rich women during liposuction to manufacture expensive beauty soap and redesigns airline emergency books to hilariously offensive effect.

Above all, Tyler preaches a philosophy of returning to the primitive. 'We're a generation of boys raised by women,' he says. 'Our Great War is a spiritual war, our Depression is our lives. Self-improvement is masturbation, self-destruction might be the answer', and he suggests that the way to restore masculinity is less through aggression than through the willing absorption of pain.

He and the Narrator form the clandestine Fight Club in the cellar of a rundown bar where men can batter each other senseless in bare-knuckle contests. One is reminded of the masochism of Hemingway, Philip Marlowe and those endless scenes in films from the Fifties and Sixties where an unresisting Brando is beaten to a bloody pulp. The punishment Fight Club members receive would, of course, be too much for a punch-bag, let alone a person.

The Narrator thrives in this new world, gaining a new purchase on life. But the smile is gradually wiped off his bruised face as it is off ours. First, Tyler becomes Marla's sexually athletic lover, and her presence exposes the homoerotic aspect of the ménage. Next, Tyler starts recruiting shaven-headed, black-shirted zealots from the Fight Club to form an underground army. The Narrator becomes increasingly alarmed at what he has unleashed when he learns about Project Mayhem, Tyler's all-out guerrilla war against capitalist society,

Although it falters a little towards the end, Fight Club is a dazzling and disturbing fable about the discontent of men at the end of this terrible century, and it satirises the fantasies we have of achieving a different, supposedly more natural life. Some of those fantasies have been harnessed as reality, most especially in Nazism and other movements devoted to a politics of the blood.

It's a dangerous and unnerving movie because, while they are manifestly defending civilised values, Fincher and company involve us at a visceral level in the seductive attractions of what the film attacks. It's like having a bad dream in which you go out for a night on the town and find yourself marching in a brown shirt at a Nuremberg rally.

What the censor says

Since Philip French saw Fight Club, the British Board of Film Classification has chosen to cut two scenes from it. Robin Duval, Director of the BBFC, explains that decision: 'There were two scenes that were cut in the British version: in the first Brad Pitt is beaten up at length. The second is where Ed Norton beats up a blond boy to ruin his looks. We're quite stern with the 1968 guidelines on promoting sadism as a source of pleasure. Indulging in the pleasure of violence is relatively rare, but here the point was made and then followed by a long scene where a helpless victim has his face smashed up.'