The future. Earth is in a state of war with an alien race who kidnap humans and replace them with human-looking walking bombs. Spencer (Sinise) is accused of being such an impostor and goes on the run.
Impostor begins well enough, but soon degenerates into a tedious, barely by-the-numbers chase movie. Gary Sinise (generally a fine actor but not a man to trust with science fiction - see Mission To Mars) is plank-rigid as the man on the run, while Vincent D'Onofrio hams it up desperately as his government-sponsored nemesis.
Originally part of an abandoned portmanteau of short sci-fi movies (Alien Love Triangle, the title chapter of which was directed by Danny Boyle), the film shows clear signs of a traumatic time being retrofitted in the editing suite. It isn't a total catastrophe, however. At moments it looks impressive, particularly in its shots of Earth's cities, and some of the set design is imaginative. But otherwise it's just a refugee from the DTV bin masquerading as a cinema release.
There are the vestiges of a cool idea here - no surprise since it's based on a story by the late Philip K. Dick but it's squandered in a miasma of indifferent screenwriting and wobbly performances.