A Scanner Darkly

Fred is an undercover cop, he protects the good people from the plague of drug addicts hooked on Substance M who make the city an unsafe place. But in order to carry out his work, Fred must become Bob Arctor, the owner of a house where he lives with two other addicts who, like him, spend their days rambling loudly and frying their brain with all kinds of illegal substances. But... is Fred impersonating Bob Arctor? Or is it Bob Arctor impersonating Fred?