It's the year 2079 and Earth is under attack from the Centaurians, invaders from a distant solar system that want to conquer Earth. As a result, most well-to-do Earthlings live under protective electromagnetic domes that shield them from the relentless attacks, while agents from the Earth Security Agency, such as Major D.H. Hathaway (VINCENT D'ONOFRIO) of the enemy infiltration unit, are on the lookout for synthetic human "replicants."
They're beings who look, act and think just like the real people they've replaced, unaware that they're really assassins equipped with bombs in their chests programmed to kill important political figures. Hathaway believes that Spencer Olham (GARY SINISE) is one of them who's targeted to terminate a visiting Chancellor. A respected scientist with the Defense Bureau who's designed a massive and potent military weapon with his friend and colleague, Nelson (TONY SHALHOUB), Spencer can't believe the accusation and claims he's the real person.
Hathaway disagrees and is ready to kill him to remove the bomb. Thus, Spencer escapes and suddenly turns into public enemy number one. He ends up in the Dead Zone outside the protective city dome and finds himself at the mercy of street dweller Cale (MEKHI PHIFER), who agrees to assist him in exchange for access to valuable pharmaceutical drugs.
As the unlikely partners make their way back into the city and to the hospital where Spencer's wife,
Maya (MADELEINE STOWE), works as a doctor and administrator, Spencer hopes to run a medical scan that will prove his innocence, all while trying to avoid Hathaway and his troops who are determined to catch and/or kill him.