It's hard to put into words the genuine shock to the system that Gasper Noé's now notorious Irréversible delivers. There may well be a fair number of you out there who have never seen it and even a few who have never heard of it. There are also, I would imagine, a sizeable number who know all about it and have no wish to watch it, and probably just as many who have seen it once and have no desire to sit through it a second time. I can understand that. I have friends for whom extreme cinema is part of their staple diet but who shake their heads and suck air through their teeth when you bring up Irréversible.

For the few of you who are new to the film, Irréversible is reverse-structured rape/revenge story, a new take on a sub-genre that regularly attracts charges of exploitation, which is hardly surprising given that two of its most famous torch-bearers are Wes Craven's 1972 The Last House on the Left and Meir Zarchi's 1978 I Spit on Your Grave. But if Zarchi's film is often cited as the genre's nadir, Craven's is generally regarded as a deeper work than the surface horror suggests and was actually a partial remake of Ingmar Bergman's 1960 The Virgin Spring, perhaps the unwitting godfather of a subgenre that Bergman himself would doubtless have shuddered at the idea of being even tentatively associated with. Balancing exploitation with more serious concerns has been the difficulty for any filmmaker wandering into this morally dubious area, from Sam Peckinpah's 1971 Straw Dogs to Abel Ferrera's 1981 Ms. 45 and Tony Garnett's 1983 Hand Gun. All have their enthusiastic supporters but also a number of very vocal detractors, and it's easy to see why – the victims are usually female, the filmmakers male, and any trivialisation of a crime as wretched as rape usually deserves every brickbat thrown at it. That few of us have the same reaction to on-screen murder perhaps (and I do mean perhaps, as I know there are other factors at work here) says more about its cinematic frequency than it does about how seriously we regard the act itself.

If opinion was divided on the above named films, then it was complete polarised by Gasper Noé's Irréversible. Noé himself is no stranger to controversy – the central character of his 1998 debut feature Seul contre tous was a sexist and racist homophobe whose spleen-venting monologues and brutal assault on his pregnant wife make him one of the most unlovable anti-heroes in modern cinema. Irréversible sparked an even deeper critical divide, accusations of attention-grabbing exploitation and a childish determination to shock at all costs going head-to-head with praise for the film's daring, intelligence and thunderous power. This battle of opinions was given lucid voice in the February 2003 issue of Sight & Sound, in which critics Nick James and Mark Kermode justified in some detail their opposing standpoints on the film.

So what exactly is the problem here? The debate, for those who do not know, revolves largely around two genuinely horrifying key scenes. The first occurs just minutes into the film, when a man has his head beaten to a bloody pulp with a fire extinguisher, an assault whose extended viciousness goes way beyond its initial purpose of instinctive retaliation for violence inflicted on the attacker's friend. The first blow sees to that – the subsequent twenty-one are about something else entirely.

The film then shows us the immediate build-up to this moment. We know little about any of the characters at this point, save that two of them are urgently searching a demonically lit gay nightclub known as Le Rectum for a man nicknamed Le Tenia. When the more aggressive of the pair angrily confronts the man he believes is the object of his search, a fight breaks out and he is pushed to the floor and his arm is broken. He is on the verge of being raped when his seemingly more reluctant companion steps in and does extreme violence.

If you don't know at the start (and few will approach the film without some foreknowledge of this), then it won't be long before you realise that what immediately follows is not a flashback to clarify the opening attack, but the next sequence in a reverse narrative of the sort made famous by Christopher Nolan's Memento. In the manner (but definitely not the style) of a news report, we stumble onto an incident whose back story is gradually revealed through cinematic investigation. The guy with the broken arm is Marcus, and he's riding on fury – on his way to the club he assaults a taxi driver, loses his rag in a café and smashes the windows of a car to stop his friend Pierre from driving off and leaving him. Pierre is increasingly revealed as a somewhat unwilling accomplice, repeatedly trying dissuade his angry companion from doing anything rash. "What's your anger about?" he asks him. "Fucking B-movie revenge crap!"

We're quite a way back into the story when we get the first real window into the source of Marcus's fury. Someone has been raped, we are told, and it's clearly someone he knows well. Another hop backwards and we find the two men in shock, reacting to what we (and they) have yet to witness and being propositioned by a local gangster who calmly assures Marcus that "until proof to the contrary, I can be your best friend." This new friend knows the name of the man who committed the crime that they are so shaken by, and for a fee promises to help make him suffer. Hop back again and Marcus and Pierre are still happy, but not for long – they emerge from a party and almost immediately encounter the end result of the incident that will set them on the path to vengeance. By now we know the nature of the crime and have seen its terrible consequences. Are we apprehensive? You don't know the half of it.

As the next jump back begins, the camera follows an unidentified woman down into a red-walled subway, and there will be few who will not fear what is to follow. And I can pretty much guarantee that it's far worse than anything you will imagine or expect. This scene has been the key focus of discussion, disagreement and audience horror since the film first screened at Cannes back in May 2002, and I would imagine at least partly responsible for its reputation as one of the most walked-out-of movies in modern cinema – two hundred got up and left during the Cannes screening alone. It is, without question, an almost unbearable scene to watch, as a repulsively brutal rape that is shown in its entirety and in real time as a single, static shot held for agonising length, and is followed by a beating violent enough to make even the most hardened viewer recoil.

The argument against the film can certainly be said to have ammunition here. On the surface at least, this does appear to be designed purely to out-shock everything that even extreme cinema has given us before, guaranteeing the film the sort of press coverage that will find it an audience based on the controversy alone. And, of course, there's the gimmick of the backward running narrative, and I haven't even mentioned the wild, disorientating camerawork and the anxiety-inducing soundtrack. Exploitation cinema at its most unredeemable, then? It's certainly an easy case to make based on the evidence as listed, but to do so entails shutting your eyes, ears and intellect to Noé's clear sense of purpose, and to the film's very considerable achievement both as drama and as cinema.

The reverse narrative structure is purposefully employed, inverting the usual dramatic set-up of introducing the characters first and the narrative disruption later, and in the process undermining the standard structure of the rape/revenge subgenre itself. Here revenge provides no visceral thrills or moral satisfaction because we have no emotional investment in the characters at this stage. Later, we can look back and perhaps feel that Le Tenia got what he deserved, but you don't get to enjoy the vengeance in the usual manner, and a second viewing reveals that's it's not him that takes the beating anyway but an unfortunate associate, one whose murder Le Tenia watches and appears to enjoy.

If the rape itself is sickening to watch then it damned well should be, and though you'll want to turn your head away, you shouldn't. It has been suggested that men find the scene even more unbearable than women, as they are forced into the situation of identifying completely with the female victim and understanding just something of the sheer awful horror of what it is to be so assaulted (the gender barrier to identification is further destroyed by the nature of the attack itself, which could just as easily have been inflicted on a man). So unwavering is the camera's stare, so real does Alex's suffering feel, that we as an audience become inevitably complicit, unable to intervene, to respond to the hand that reaches out desperately for our help. I swear you'll never hear the word 'rape' again without recalling this scene, and you'll never hear it taken lightly again by anyone without wanting to subject them to the Ludovico Treatment with this sequence as the medicine.

The camera at times feels chained to the emotional condition of the characters it observes, the frantically mobile, almost unframed insanity of Marcus and Pierre's arrival at Le Rectum settling down over the course of the film to an eventual stedicam calm as we and they are transported back to less troubled times. Elsewhere it echoes the rhythm of an anguished heartbeat, or drifts around rooms or between locations like a half-formed memory, an experience captured only in essence rather than in fine detail.

The editing is largely invisible, the impression being of a film told in a single wandering shot, short chapters digitally linked to allow the camera to fly between moving vehicles, and in and out of closed windows like a curious insect. The aplomb with which the camera operator (usually Noé himself) and the actors pull off the largely unscripted ten to fifteen minute sequences is quietly mesmerising. The most complex of shots is so casually executed that it's only afterwards that you realise that we have followed the three leads down into a Metro station, onto a train and three stops down the line to their destination in one unbroken ten minute take, with them confidently and entertainingly improvising dialogue the whole way. Sound plays an equally important role in grinding the opening scenes into the brain, a single electronic chord that builds in volume and intensity and is underscored by low frequency rumble pitched at 27hz, the sort of half-audible tone that has been proven to induce anxiety and even nausea in those who hear it. Used in some countries by riot police to disperse crowds, this technique has been credited with as many walkouts as the rape scene.

By inverting the narrative, Noé robs the film of a traditional climactic scene, instead providing a compelling backward journey from darkness into light, a serenity that is nonetheless tarnished by the foreknowledge of the fate that is soon to befall characters we by then know and care for. Twice we are told that "time destroys all things," a truism of the cycle of nature that is here robbed of the optimism of rebirth and evolutionary progression. Or is it? The narrative structure suggests that the future is already written and that we are prisoners of fate, and your response to the ending will depend on whether you share this viewpoint. I do not. The very idea of a pre-determined future absolves everyone of responsibility for their actions and makes a mockery of free will and even the wonders of chance, and chance is as much responsible for Alex's fate as malicious intent. With that in mind, and with infinite possibilities for any future, the ending of Irréversible can be seen to shine a ray of hope on its own searing vision of a dark and unpleasant world.

There may still be issues to answer here, not least what could be read as homophobic overtones (Noé's cameo as a masturbating man in Le Rectum is reportedly due to his own concerns over this possible perception), but in the great debate over Irréversible's merits or otherwise, I am fully prepared to shout in its support. Although as difficult and challenging a film as you'll find on DVD anywhere at the moment, it's also a bold, fiercely performed and brilliantly executed work that genuinely assaults the senses and, for those with the stamina to see it through, really does reward the intellect.

Shot on Super-16 and transferred to High definition video for scope reframing and effects work, the picture was always going to display a certain amount of grain and it's very visible here, but it also works fine for the film's initially demented aesthetic. Contrast and black levels are bang on, the lost-in-the-dark arrival at Le Rectum clearly intended to be disorientating and unclear. The very specific colour palette also comes across well, though the use of dark red tints works better in a darkened room than in bright sunshine. A fine transfer that is framed 2.35:1 and enhanced for widescreen TVs.

The usual Tartan trio of soundtracks are on offer, but good though the stereo 2.0 is, 5.1 and DTS are the only way to go if you want to experience the full effects of Thomas Bangalter's music and Noé's genuinely unsettling use of low frequency rumbles. A terrific soundtrack that is organically integrated to the visuals and the emotions of the characters, providing a genuinely nightmarishly audio-visual experience.

One thing the original region 1 release always had over its UK cousin was a commentary by director Gasper Noé. Now Tartan have gone one better in this new Collector's Edition with a director and cast commentary, featuring Noé and actors Monica Bellucci and Vincent Cassel. Not bad, huh? Except that despite this claim on the extras menu, that's not we we get. Noé is certainly on board and has plenty to say, but there is no sign of Bellucci or Cassel, a damned shame when you consider how interesting Bellucci's views and memories of filming the rape scene would have been. But this blip should not count as a deciding factor on whether to buy, as Noé is so bristling with information that I did not for a second feel short-changed. He discusses his influences, the cast, the soundtrack, the filming of specific sequences, the ingenious use of digital effects (this was especially eye opening) and a whole lot more. It's an essential and consistently enthralling companion to the film.

SFX (7:12) is a short featurette of the digital effects work done for the film by visual effects studio Mac Guff, and based around an interview with effects man Rodolphe Chabrier. Plenty of before-and-after examples are included, and it makes for very revealing and fascinating viewing. I could have done with more on this.

Intoxication (4:56) is an interview with director Stéphane Druout, who appears in the opening scene opposite Philippe Nahon (replaying his character from Noé's Seul contre tous). There's a melancholy feel to the chat, which is conducted in Druout's dingy kitchen to the sound of rainfall and kicks off with him taking medication to help him cope with the symptoms of AIDS.

Stress (4:32) and Outrage (4:24) are music videos made up of drifting hand-held footage from two of the film's locations, set to electronic tracks by Thomas Bangalter, half of Daft Punk and the man responsible for the film's score.

Teasers (3:09) consists of 6 short teaser trailers, three of them semi-abstract, one a little misleading and two that point you in the right direction.

The Trailer (1:42) is well assembled, but doesn't provide any real warnings of things to come.

Many will already have made up their minds about Noé's film, and if you hated it then I concede that none of the above is likely to persuade you to take another look. If you haven't seen it then you should by now at least be aware of the horrors that lie ahead and will approach it with your eyes open (if you can keep them that way during the strobe lighting assault at the very end, you might just find yourself twitching on the floor). For those who already appreciate Noé's considerable cinematic achievement, then Tartan's re-release really delivers. The picture is good, the soundtrack brain-rattling, and although the actors are absent from the commentary, it's hard to imagine that Noé would have let them get a word in anyway. Highly recommended, but with the obvious warnings.