The film is set in California "seven years in the future", when the war on drugs has been lost in the USA and a significant proportion of the population are now addicted to a drug known as Substance D. Bob Arctor (Keanu Reeves) is an undercover detective working to infiltrate the world of addicts and dealers, his identity protected from his colleagues by wearing a hi-tech scramble suit and adopting the alias "Officer Fred". As Fred, he is assigned to focus his attentions on Bob Arctor, thereby having to spy on himself, all the while regularly taking Substance D, which increasingly affects his ability to keep track of both personas.
The premise of A Scanner Darkly is undoubtedly complex but holds the potential to be incredibly compelling. It's a shame then that the film largely falls short of fulfilling its promise. The dual character of Arctor is a demanding role, and it's one that Reeves never manages to pull off. When soliloquising as Arctor, there is very little hint that the two sides of Arctor's life and personality are conflicting internally; when he shares scenes with other characters, Reeves simply merges into the background. Not something you want from your main character. Much of the film's success hinges on our understanding of Arctor and our ability to follow both sides of his personality; Reeves comprehensively fails to make this happen, giving a performance that is decidedly wooden.
The supporting cast do better: Robert Downey Jr. as Barris is a textbook arrogant blowhard who knows a lot less than he thinks he does, creating a vile yet compelling character; Woody Harrelson's turn as likable laidback addict Ernie provides some of the most successful humour throughout; and Winona Ryder as small-time dealer Donna, whilst not as strong here as Harrelson or Downey Jr., provides pleasing support.
It's a shame then that, for large parts of the story, these potentially compelling characters don't have a huge amount to do. Many of the scenes boil down to simply listening to drug addicts talking to each other whilst under the influence which, in truth, really isn't that interesting. When the action isn't following the collective Arctor is simultaneously a part of and spying upon, it focuses upon Arctor's work life away from the group. Aside from some neat touches with near-future technology, this again isn't all that interesting, something which Reeves' flat performance only highlights further. By the time the final act kicks in, the plot progression feels rushed and the moral message somewhat tacked on.
A Scanner Darkly does have some redeeming features however. There are a handful of comedic scenes which are immensely successful - the group's drug-influenced investigation into whether a recently purchased bike has any "missing gears" is genuinely brilliant, and the surreal narration of the botched suicide attempt of Freck (Rory Cochrane) is blackly inspired. The "rotoshop" style used throughout is also impressive, offering the film a pulpy graphic novel feel. The style is also used to great effect at times; it's hard to imagine the depiction of the scramble suits used by Arctor and other characters in the film being quite as effective through traditional live-action filming.
What we end up with is a mixed bag, and certainly not the successful Dick adaptation many had hoped for. Whilst there are things that make A Scanner Darkly worth a watch, the flaws are too hefty to truly recommend it. It's so-so at best, and ultimately a wasted opportunity.