How do you prepare an unsuspecting audience for a film like Decasia? You can't outline the plot, because there isn't one. You cannot examine key points of the narrative, because none exists. Discussion on character development is pointless, because although a variety of people are on display, we never know who they are, and nothing they do affects us on an emotional level. Most of the time, we can barely make out even what they look like, and we never care about them, nor are we meant to. They are not important, not in themselves. They only function as shapes in the background, objects that have been manipulated by time and decay. You think Mel Gibson is being difficult by filming The Passion of The Christ in Aramaic? Well you definitely ain't ready for this.

Imagine a film with no story, no cast list, no dialogue, and that is made up completely of badly decayed nitrate film clips. But that's not all. Despite the varying content of the film extracts, this is not about the images on display, but the decay itself, the damage that the film has suffered over during its decades of storage in locations that were sometimes far from ideal. Having sat through untold film prints in less than perfect condition and winced at the scratches and fluff and damage that build with every reel change, I was being asked to engage with a film in which that very damage is the film's sole raison d'être. Are these people kidding?

It was the most electrifying, hypnotic experience I have had in a cinema all year.

I know that no matter how hard I try to sell this, only a tiny minority of the film's potential viewers would even think about handing over money to watch this film, and of those who do a fair proportion will be out of the cinema before the first twenty minutes are up. If the images don't get you then the music will - Michael Gordon's score is unsettling, eerie and sometimes thunderous, but never comfortable. It's a tour-de-force in its own right, but working in conjunction with the imagery is an all-out assault on the senses, a sometimes confrontational and, at the right volume, ear-battering minimalist symphony that is rarely less than aggressive, and is never a safe listening experience - even at its most rhythmically melodic, the tuneful tinklings in the foreground are undercut by an off-key whine of strings behind.

The imagery itself is at times genuinely mesmerising. Even the more extensive film damage does not completely obscure the original content, and in the more extraordinary moments appears to interact with it - in one clip a boxer punches into a pool of flickering destruction, while in another the horizontally rotating rockets of a fairground ride are propelled from a rapidly mutating cloud of film damage that looks almost like a gateway to another dimension. Elsewhere the damage has had a most unexpected effect on the imagery: buildings, cars and faces collapse into a liquid form that ripples and oscillates like a reflection in water under fire from a sonic death ray; nuns watching over a group of schoolchildren are propelled from positive to negative and back, visiting a wide variety of solarised places between; as a man is rescued from drowning, the entire frame is assaulted by large black circles of decay like some nightmarish radio-active rain. The damage never flits past as it would on a modern, 24 frame-per-second feature, step printing ensuring that every spot of decay registers and provides the imagery itself with a rhythm that seems perfectly turned to the kinetic heart of the musical score.

You can't help but think that, given a couple of years the right post-production software and the patience of a saint, Morrison could have taken stable archive footage and produced the images seen here to order, but this would remove the random element that makes the film such unexpected and sometimes startling viewing. Morrison examined almost a thousand prints from a variety of sources to make his selection, but exactly what has guided his specific choice of imagery and editing decisions can only be speculated on - shots of what appears to be a genuine mine rescue sit alongside travelogue material from the Middle East and unidentifiable drama footage. Morrison himself has talked about being drawn to "examples of man defying his own mortality," but ultimately you feel that it is the tone and feel of the clip that governs its selection, and how it reacts with (or against) the music it is being set to. At times this wedding of sound and vision touches perfection and the effect can be genuinely overpowering - on occasion it provoked in me a complete sensory overload. During one unexpected change of tone in the score, when the bass suddenly thumped through my chest and the screen was awash with oscillating boulders of damage, I genuinely thought, sitting as I was in the front row, that my brain was going to explode.

Setting abstract images to music goes all the way back to 1940 and Disney's Fantasia and comparisons will inevitably be made to Godfrey Reggio's 1983 Koyaanisqatsi, but the similarities here are on a superficial level. In Koyaanisqatsi, Ron Fricke's arresting images and Philips Glass's melodic score are ultimately very user friendly, caressing the eyes and ears in a way that bears little resemblance to the audio-visual smack up the senses offered by Morrison and Gordon. And despite its rejection of narrative, Reggio's film makes a clear point about our modern state of living, whereas Morrison's film is, as they say, about the art and the experience alone. Ultimately Decasia is a gallery piece, an artwork targeted at the senses and a part of the brain that no standard feature film is ever going to touch, but the effect on a receptive audience is inevitably going to be an emotional one, the result of having collectively experienced something so extraordinary and unique. At the screening I attended there were plenty of walkouts (reasons given ranged from boredom to simply being overwhelmed by this audio-visual assault), but at the film's conclusion those who stayed were looking round at each other with dropped jaws and widened eyes.

I genuinely cannot imagine seeing this film on DVD or video - the images have to fill your field of vision on a screen at least 10 metres wide - and was reminded of my first visit to the Tate gallery in London, when I walked into a side room and was confronted with the sheer, awesome scale of Monet's Water Lillies, a comparison I do not make lightly. Since the beginning of cinema the arguments have raged over whether film can possibly be considered as art. In the hands of Bill Morrison, no other classification will adequately suffice.