Where does its power to unsettle come from? It isn't just violence or foul language or cruelty or sexual perversion (although it has all these in abundance). It's a film that actually gets in and messes with your mind.

Blue Velvet, like the most powerful and enduring folk tales, is a huge allegory, ripe with symbolism and suggestion, full of archetypes and universals drawn from our group unconscious, the place where our deepest fears and loathings and our secret desires and guilty passions and obsessions have been carefully hidden away. It puts forward a view of the individual, and by extension of human society, to the effect that what you see is not what you get.

Lumberton, in the film's opening scenes, is presented as idyllic Middle America; fine, friendly hard-working people, white picket fences bright in the sunlight, flowers blooming in the well-tended gardens, their saturated colours almost dazzling to behold, a vision of small-town normality and wholesomeness. A smiling fireman on the running- board of a gleaming red fire-engine waves cheerily as he goes by. Then, misfortune intrudes: the man hosing his garden has a seizure or stroke of some kind, symbolised by the kink in the hose that stops the flow of water, but it is a natural, biological event without moral significance. However, as he lies in his helpless agony, tended only by the dog and the little toddler, the camera zooms down to the grass, and to what is going on beneath the grass, in a shot that I think provides the key to the entire film: for just below the surface sickening and unspeakable things are happening - ugly and sinister bugs are tearing one another apart. There are violent and repellent things going on just out of sight, things we will never know about unless we seek them out, unless we dig. It is an image, I think, that we are meant to apply to ourselves as much as to Lumberton, or any other version of a human society. It is the film's overarching metaphor.

The development of the story involves the finding of a severed human ear by the central character Jeffrey (Kyle MacLachlan) and his attempts, with the help of High School student Sandy (Laura Dern) to unravel the mystery behind its amputation. This quest turns into something closely resembling an archetypal fairy-tale, in which he finds the beautiful woman Dorothy (Isabella Rossellini) being held prisoner in a grim and forbidding tower (apartment block) by the ogre-like incarnation of pure evil Frank (Dennis Hopper) who is involved in drugs, kidnapping, prostitution and almost certainly murder, and who is subjecting her to the most bizarre acts of torture and sexual humiliation. Dorothy is eventually released from her nightmare existence, though not directly by Jeffrey, and not without suffering an almost total breakdown which leaves her wandering naked through the town's darkened streets. So we might want to say that this is no straightforward fairy-tale, but rather it is one that turns sour, becomes corrupted and debased. It is a fairy tale whose "happy ending" still leaves us with a sickly feeling in the pit of our stomach.

To rescue the damsel in distress the hero in the fairy-tale of old usually had to go through some pretty extreme tests and adventures, and Jeffrey's lot is no different. His decision to become the "bug man" (literally as well as metaphorically) and to root-out the horrific evil that is holding Dorothy in its grip takes him to the very depths of the hell that is Frank's universe. It would be difficult to imagine anyone more evil than Frank, or events more spine-chilling than the goings-on at the brothel where he is holding Dorothy's son and her husband prisoner. Once or twice the limits of credibility seem to be stretched beyond breaking point, as when the policeman in the yellow jacket, although dead, remains standing until Jeffrey brushes against him, but overall the film manages to maintain a delicate ambiguity with regard to objective reality. We never quite know whether to believe in Jeffrey's Odyssey or not, whether to view it as literal narrative or as a dream-sequence, or as some kind of grotesque daydream / fantasy in which Jeffrey has cast himself in the role of hero but of which he slowly loses control as the wanderings of his mind become more and more fevered, forcing him to acknowledge aspects of his own personality that he would rather have left unexamined. For of course Jeffrey finds in himself the seed of Frank's sadism: most clearly when Dorothy (whose personality has also been twisted by her exposure to Frank) asks him to hit her and he obeys, and, we suspect, at some level enjoys doing it. Jeffrey's shining armour dulls considerably in the course of his journey through the underworld: his self-image undergoes what we take to be a permanent change.

We are probably mistaken if we accept very much of David Lynch's material at face value. There are certain shots which he seems to use in a very peculiar way, as markers, or even quotation marks, to plant clues as to where we should adjust our perceptions. The whole quasi-dream-sequence involving Frank and Dorothy is contained between two near-identical zoom shots into and out of a human ear. These must signal something special about this part of the narrative, but what exactly the "specialness" is Lynch is happy to leave to our individual interpretation. In a parallel way he uses the same shot of the waving man on the fire-engine both at the beginning and the end of the film, like book-ends to the entire work. Is he trying to suggest that perhaps no time has passed but we have simply taken a diversion into an alternate reality and returned to where we started, or even that this apple-pie-America world is a sham like the one inhabited by Jim Carrey in The Truman Show fourteen years later, or perhaps that it is just as much a conceptual make-believe as the one in which Frank mimes to the Sandman and enjoys his drug of choice through a face-mask while he sexually assaults his victims. I personally favour this latter theory. Lynch is saying to us: "Don't kid yourself, none of this is real. We see the world the way we want to see it, the way we expect to see it. You wouldn't want to know what it's really like".