To the academic world's small population of postmodernists, Slavoj Zizek - a shambling, rambling Slovenian philosopher - is a folk hero. At any lecture podium, any time, anywhere, he will emit hazy clouds of gaseous theory with the speedy intensity and comic riffs of Bill Hicks.

He seemed to emerge fully formed from the wreckage of the former Yugoslavia with an ec lectic magpie-philosophy, rapidly spewing out books and essays on everything from opera to the use of torture in the TV series 24. Zizek is the biggest box-office draw postmodernists have ever had, their best punch at the bestseller lists. The press fawns upon him; he has been called an "intellectual rock star"; and, according to a recent profile in the New Yorker, Slovenia has a "repu tation disproportionately large for its size due to the work of Slavoj Zizek".

In the opening scenes of Zizek!, a new feature-length documentary, it is not hard to see why they fall for him. Zizek looks like an immense human Droopy Dawg. He talks with such babbling, neurotic force about everything from quantum physics and Hegel to Meg Ryan that, for a moment, he is hypnotic. Leading the film-makers through his chaotic transcontinental life, he jabbers to them from his bed and even takes them to a long staircase where he fantasises about killing himself - before posing as a splattered corpse on the concrete floor beneath.

As the film progresses, however, Zizek does more than symbolically enact his own death; he commits intellectual suicide, all but admitting that his "philosophy" is a slew of nonsense. If the director, Astra Taylor, intended to make a fawning fan letter - as her cameos in the film suggest - she has failed. If she intended to shred Zizek's credibility, she has succeeded stunningly.

What does Slavoj Zizek believe? What does he argue for? Such obvious questions are considered vulgar among postmodernists. When you first look through the more than 50 books he has written, it is almost impossible to find an answer. It seems he seeks to splice Karl Marx with the notoriously incomprehensible French psychoanalyst Jacques Lacan, slathering on top an infinite number of pop-cultural references.

His defenders claim he is trying to stretch the scope of philosophy to cover the everyday flotsam that philosophers have hitherto ignored. But gradually, as you pore through Zizek's words or watch his audiences, whose bemusement is caught on film, you discover that the complex manner in which he expresses himself does not imply that his thought is itself subtle or complex. In fact, he seeks to revive a murderous and discredited ideology.

Asked by an audience member what his idea of a good social order is, he replies: "Communism! I am absolutely in favour of egalitar ianism with a taste of terror." Behind Zizek's comedy routines, he believes we need to return to Bolshevism. He is not offering warm, fuzzy Lennonism; this is cold, bloody Leninism. Zizek writes rapturous hymns of praise to the "genius" and "strength" of Vladimir Ilyich Lenin, calling him "the poli tician of the 20th century" and demanding "fidelity to Lenin's legacy". Just in case there is any ambiguity about the anti-democratic nature of supporting the man who erected a monstrous one-party police state in Russia, Zizek explains that Lenin's "ultimate lesson is that only by throwing off our attachment to liberal democracy" can we become virtuous.

This contempt for liberal democracy and preference for dictatorship is a constant in Zizek's work. He approvingly quotes Alain Badiou, who argues: "Today the enemy is not called Empire or Capital. It's called Democracy." Zizek says about Benito Mussolini: "You know, the democrats in 1925 accused Mussolini: 'You want to rule Italy, but you don't have any programme.' You know what was his answer? 'We do have a programme: our programme is to rule Italy at any price.' I love Mussolini."

When in the mid-1990s, the Slovenian prime minister asked Zizek if he wanted to be a government minister, he replied: "I am only interested in two posts - either minister of the interior or head of the secret police." He condemns the language of human rights as an unacceptable brake on reconstructing Leninism. Asked about Stalin, he says: "My big worry is not being ignored, but to be accepted. People still have this idea that this guy did some bad crimes . . . It's not as simple as that - that I am simply a Stalinist. That would be crazy, tasteless, and so on. But obviously there is something in it, that it's not simply a joke."

He praises Mao Zedong's notorious indifference to the potential large-scale loss of human life in a nuclear war as "a cosmic perspective" and a "message of courage". He says the "terror" involved in Maoism is "nothing less than the condition of freedom".

When you peel back the patina of postmodernism, there is old-fashioned philo-tyrannical nonsense here. At some level, Zizek knows this is preposterous; he lived under Soviet tyranny, and even joined the opposition. Simply by putting a camera in front of him and leaving it running, Taylor shows how his façade and his ideas are crumbling. After insisting that his claim to be a Stalinist "is not a joke", Zizek suddenly admits: "I think there was a thing called totalitarianism, and it was bad . . . You know, if I was not myself, I would arrest myself." He then admits that his political positions are monstrous: "The worst thing is to play the 'we are all human' game. I am not human. I am a monster. It is not . . . that I wear the mask of a theoretician and underneath I am a warm human being. I am a monster who plays, pretends he is human."

Zizek expresses this monstrousness repeatedly in his writing, mocking liberals who shy away from the "cruelty" necessary to build his ideal world. He recounts with admiration this anecdote: "Walking to his theatre in July 1956, Brecht passed a column of Soviet tanks rolling towards the Stalinallee to crush the workers' rebellion. He waved at them and later wrote in his diary that, at that moment, he was for the first time in his life tempted to join the Communist Party." Zizek calls this "an exemplary case of the passion of the real. It wasn't that Brecht supported the military action, but that he perceived and endor sed the violence as a sign of authenticity."

So is Zizek a kind of philosophers' Borat, taking ludicrous positions to see how far he can push them? His followers dismiss every depraved political statement as an ironic joke. At times he insists he is not a comedian, that he means every word. Then he confesses in a moment of self-awareness: "My eternal fear is that if for a moment I stopped talking the whole spectacular appearance would disintegrate [and] people would think there is nobody and nothing there. They would think I am a nobody who has to pretend all the time to be a somebody."

As he watches his hero Jacques Lacan deliver an incomprehensible lecture on video, Zizek exclaims: "There is nothing behind this obscurity. This is just bluffing." It is a plain moment of projection, and an unwitting confession of charlatanism. His political thought quickly descends into contradictory drivel, where he claims he is against the people who condemn the bombing of Kosovo and against the people who condone it, and calls for "a revolution without revolution". He has, of course, constructed a convoluted epistemology to justify this, claiming that, in reality, "we can only speak about things that do not exist" and "we can ultimately only talk . . . about things we do not understand".

This kind of thought can only be entertained because nobody would ever take it seriously enough to act on it. When Gilles Deleuze and Félix Guattari say we should all become schizophrenic, when the gay Michel Foucault embraces the murderously homophobic Ayatollah Kho meini, when Zizek suggests a return to Leninist terror - these very positions are admissions that postmodernism is merely an unserious confection by intellectuals. It leads nowhere except to demoralisation and disaffection.

Zizek! is a painful film, almost the record of a philosophical nervous breakdown. You do not end up hating Zizek, not even when he says with Stalinist relish that he wants to rehabilitate "notions of discipline, collective order, subordination". Rather, you end up hating the academics who take this non-thought seriously. Are they really saying you can advocate tyranny as long as you throw in a few gags about Keanu Reeves? In the end, they leave us nothing but a theory-clown with bloody tears.

"Zizek!" (unrated) is released on 4 May