Adapter/director Richard Linklater achieved at least three remarkable things with A Scanner Darkly: he created the most faithful movie adaptation of a Philip K. Dick novel or short story ever; he created an outstanding science-fiction film; and he maximized the limited acting ability of Keanu Reeves by casting Reeves as a burnt-out case in the midst of a drug-fueled mental breakdown.
Reeves plays Bob Arctor, a near-future California undercover government narc charged by his superiors with helping win the war against Substance D, a highly addictive illegal substance that rapidly causes irreversible brain damage in those addicted to it, partially by severing the connection between an addict's left and right brain hemispheres.
Arctor is deep undercover, sharing a house with two other addicts and buying Substance D from a third in increasingly difficult-to-supply mass quantities in the hopes of moving up the supply chain. The government knows what the main ingredient of Substance D derives from -- a small, blue-flowered plant -- but it doesn't know who is growing it, refining it, and putting it on the street.
Dick based much of A Scanner Darkly on his own drug experiences of the 1960's and 1970's, experiences which saw him committed to a mental asylum for a time, and experiences which caused him to interact with a large number of doomed and mostly doomed addicts. Indeed, the movie appends a portion of the novel's afterword to the end of the movie -- a roll call of the dead and damaged.
The hyper-colourful, rotoscoped animation Linklater uses here (he first used it in Waking Life) suits the material and the tone of that material -- the movie looks like a fever dream, a pulsating nightmare in which nothing is stable. All the principals deliver outstanding performances, including Reeves, and perhaps most notably Robert Downey Jr., who presents us with a jittery speed freak (Substance D appears to be at least partially an amphetamine) over-bursting with his own paranoid delusions and fantasies.
The title is a play on the Biblical phrase 'Through a glass, darkly': there are scanners in this movie, but they aren't the Cronenberg variety.