Good B-movie science fiction is like a circus high-wire act. A balance has to be struck between the focus on plot and the flashy futurism, the plausibility and the suspension of disbelief, the serious storyline and the inevitable cheese factor, the humanity and the ham-fisted acting.

Good B-movie science fiction can run the gamut from cerebral and philosophical ("Blade Runner") to serious but semi-cheesy (the original "Planet of the Apes") to borderline-ludicrous guilty pleasure ("Logan's Run"). But if the elements aren't in relative harmony, B-movie sci-fi can crash and burn like "Battlefield Earth" and "Starship Troopers."

In "Impostor" director Gary Fleder ("Don't Say A Word") does a dexterous balancing act. The story he tells has less credence than the ominous and utterly enveloping future world in which it takes place -- but he tells it with such atmospheric ballast that shrugging off the plot's persnickety problems is simply a matter of going into the movie in a shrugging mood.

A man-against-the-system action-chiller adapted from a Philip K. Dick story, "Impostor" takes place on a semi-decimated, totalitarian Earth at war with aliens from Alpha Centauri. The planet's few remaining cities survive steady bombardment under force-field domes, inside which Earth scientists have spent decades perfecting a super-weapon that could end the war, much as the A-bomb ended World War II.

The point man on the project has been Spencer Olham (Gary Sinise), who gave up childhood dreams of space exploration when his fighter-pilot father was tortured to death in a Centauri prison camp. He dedicated his scientific genius to defeating the aliens and dedicated his heart to his wife, Maya (Madeline Stowe). But after spending a weekend picnicking with Maya in the woods outside the dome, Olham finds himself on the run from government agents, who are convinced he's been replaced with an alien replicant assassin whose cloned body hides a massive bomb.

And there's a twist: if Olham is a replicant, he wouldn't know it. A "genetic cyborg evolved from synthetic DNA," he'd have been programmed with all the emotions and memories of the real Olham after he was captured and killed by the Alpha Centauri.

Any and all problems with "Impostor" stem from the fact that it is structured as one long chase scene. Each time Olham slips away from his pursuers -- led by an unrelenting and disquietingly distant secret police general (the palpably uncanny Vincent D'Onofrio) -- he does so through all-too-convenient ducts, hidden tunnels or unguarded passageways that seem to pop up whenever he's cornered. Sometimes he just ducks behind a wall and emerges somewhere else in the next scene without adequate explanation.

But while the film occasionally lacks logistical credibility, it makes up for such shortcomings with its penetrating, eerie ambiance and its tangible introspection on the meaning of humanity.

The world of "Impostor," circa 2079, is familiarly, not fantastically, futuristic. Like an extension in time from the believable near-futures of "Blade Runner" (also adapted from a Dick story) or "Gattaca," it features feasible uber-technology (TVs that spew nonstop streams of multimedia, hologram medical scans, pocket pict-o-phones) and conceivable -- not splashy -- production design. Flying transports carry people through the artificial canyons of curvilinear skyscrapers under the liquidy, translucent force field domes. Outside the domes exist both rubble-strewn wastelands and swaths of nature, each forbidding in its own way. All of this embraces semi-campy sci-fi tradition, but none of it feels laughable or far-fetched.

More importantly, "Impostor" effectively pulls the audience into Olham's predicament. It keeps the mind racing with possible escapes and outcomes while probing psychological territory with themes of paranoia and anxiety, amplified by the fact that all the citizens of this world are implanted with SIMcodes -- electronic IDs that allow the government to track anyone, anywhere. Again, hardly a fresh concept for a futuristic thriller but presented as a fact of life, not a major plot hook.

One could nit-pick "Impostor" and find many faults without much effort. But psyche is of greater consequence than intellect in this slippery science fiction shocker, and if you let go just a little logic, you'll find the film grabbing hold of you by the cerebellum. When I thought I was smarter than "Impostor" -- predicting two likely endings early in the film -- I found myself outfoxed, my expectations twisted with "Twilight Zone" finesse.