There really aren’t a lot of proper science-fiction films around – and by that I mean those that revolve around a science-based prediction of the future, rather than space fantasies of the Star Wars persuasion, or stories that crowbar one fantastical element into our current reality, like Independence Day. Even rarer are movies that take a science-fiction novel and try to do it justice, as opposed to pillaging its more immediately startling or readily comprehensible elements and ditching anything that might possibly befuddle mainstream audiences.

It’s safe to say that Philip K. Dick has suffered more than most – and perhaps it’s no real wonder. He wrote huge amounts, and was happy to sacrifice clarity and coherence for Big Ideas, leaving a lot to tempt Hollywood execs (hey, how about a film where policemen can predict any crime before it happens?) and just as much to scare them away from being faithful to the work. Even the two really good Dick adaptations, Blade Runner and Minority Report, play pretty fast and loose with their source texts.

So whatever the merits or failings of Linklater’s take of A Scanner Darkly, he deserves credit for one thing at least – he’s tried very hard to do justice to Dick’s work.

Although, going back to the start of this review, it’s fair to say that A Scanner Darkly really isn’t science-fiction in the first place. Its world of drug-addiction, of constant surveillance, of police brutality against the outspoken, was hardly a radical prediction when Dick was writing and it certainly isn’t now. This isn’t a film about how the future will change us but about how drugs – in this case, Substance D (for Death, amongst other things) – can destroy us.

Keanu Reeves plays Bob Arctor, and proves once again that if he isn’t the best actor in the world he can usually pick a good script that plays to his talents – as a character who spends half of the time out of his head on drugs and the other half investigating himself under his other identity of police officer Fred, he’s as baffled and frustrated as you’d expect. The rest of the principal cast – Winona Ryder as the dealer that Bob is buying from and Fred is investigating, and Rory Cochrane, Woody Harrelson and Robert Downey Jr as Bob’s drug-ravaged friends – are equally good, particularly Downey Jr, who seems intent on earning his recent comeback.

If there’s a problem with A Scanner Darkly it’s that most of that acting talent is spent spewing nonsense while under the influence of truck-loads of illegal substances. Linklater does a great job of drawing us into Arctor’s world but, although the dialogue is well played and frequently funny, there are also times when it seems terribly aimless. On the surface, the actual plot takes up about fifteen minutes – though the more you think about it, the more there are crucial insights to be gleaned from earlier scenes that seem fairly pointless at the time.

Linklater has not only stuck to his text but, by using an animation technique called Interpolative Rotoscoping that creates a uniquely disorientating visual effect, has tried to craft a filmic world that does it full justice. In that he’s totally succeeded. If there’s a fault here – in that watching people on drugs isn’t always that interesting – then it’s not his, unless you choose to criticise him for being too faithful to Dick’s novel.

And arguably it’s really not a fault at all. It’s just that many of the more fascinating (and disturbing) questions posed by A Scanner Darkly are only revealed by working back from the revelations of the closing minutes. Obviously anyone who’s read (and understood) the book won’t have problems, but for the rest of us it’s something approaching a chore to piece together your memories of the film to catch all the subtle nuances scattered therein. If you’re okay with that, (and don’t go expecting anything even remotely like The Matrix), then there’s a huge amount here to occupy eyes, brain, and ultimately heart.