A Scanner Darkly (15) <!-- none onestar twostar threestar fourstar fivestar -->

Films are only flickers of light, a flat parade of colours and shadows: well, of course they are, we all know that. But when we watch, the shadows and colours slip away, and we seem to be seeing real people up there in front of us, doing real things - and isn't that the whole point? In A Scanner Darkly, though, something different happens: the shadows and colours are real, and it's the people who slip away. What is the point now?

There's a clue in the source material: like Blade Runner, Minority Report and Total Recall, A Scanner Darkly is based on a novel by Philip K Dick. Unlike those films, though, this is set in a time not far from our own - "Seven years from now," the titles say - and the plot doesn't involve sophisticated hardware or a futuristic metropolis.

It is set in suburban California; Anaheim, to be precise. Bob Arctor, his housemates Barris, Luckman and Freck and his kind-of girlfriend Donna are all addicted to a drug known as "Substance D", or "death": but Bob is also Fred, an undercover narcotics cop - so deep undercover that even his boss down at the Orange County Sheriff's Office doesn't know who he is, and vice versa.

Surveillance is aided by mobile phones pinpointing your location, satellites that take snapshots from 200 miles up, computers that match face to name, address and criminal record - real life, it is a shock to realise, has caught up with Dick's fantasies, and in some ways surpassed them.

The sole item of outright sci-fi technology is the "scramble suits" Fred and his boss wear when they meet: a kind of shroud that turns the wearer into a "vague blur", with random fragments of other people flitting across - an eye, a nose, a smile passing through. The question of who is watching whom is vexed; and further so by the fact that Bob, or Fred, has been using too much D and is starting to suffer some of the side effects: the two hemispheres of his brain are no longer communicating properly, and it is a struggle for him to separate reality from illusion, dream from memory, Bob from Fred. He may be a candidate for New Path, the corporation that runs the only effective rehab programme - and, coincidentally, owns the only piece of California that isn't subject to surveillance.

The narc who gets addicted, the double agent who forgets who he's working for: nothing so new so far. What gives A Scanner Darkly its extra dimension is the way it is filmed - the way the light and colour flicker. As in his hallucinatory 2001 film Waking Life (which I was chuffed to find in my local video shop), the director, Richard Link-later, has used a process called "interpolated rotoscoping". Real actors act in real sets, then animators paint over the pictures; but, rather than paint each frame, they paint over some and leave computers to join them up.

The result is a world in which everything is unstable: shapes drift, lines wobble, patches of colour glitter and vanish. The scramble suit is a more intense expression of the way surface and substance are only approximately correlated. You catch glimpses of the people underneath the cartoon, as if live flesh is trying to break through the picture, and what we are seeing is a carapace, or a cocoon.

It's worth noting who the people beneath the cartoons are. In a film whose main subjects are surveillance and drugs, the leads are: as the shifty, manipulative Barris, Robert Downey Jr, veteran of multiple drug busts; as Donna, Winona Ryder, arrested in 2001 after being caught on CCTV stealing gear from Saks of Fifth Avenue; as the naive hophead Luckman, Woody Harrelson, an advocate of the legalisation of cannabis. By contrast, all Reeves brings to the part of Bob/Fred is a vague blur and resonances of The Matrix - another, infinitely persuasive meditation on the idea that life is but a dream. All Rory Cochrane gives to the jittery Freck, frantically scrabbling at the giant aphids that he imagines infest his hair and clothes, is acting.

But this isn't a film you'd go to see for the stars, muffled as they are by technique, or for its story. Nor, despite what his fans say, should you go to see it for Dick's quasi-philosophical musings on the nature of reality and identity - the cleverness of the apparatus with which he equipped his stories shouldn't hide the shallowness of his thinking.

If you go to see it, it should be for the atmosphere. Other film-makers - Ridley Scott, in particular - have latched on to Dick's extravagant paranoia, and the potential he offers for suspense and plot twists and looking a bit profound. Linklater, by contrast, has looked through the paranoia and uncovered the ordinary sadness and fear underneath. It is not a great film, but an insidious one, the sort that gets under your skin.It leaves you scrabbling at aphids, hoping they really are in your imagination.