The film follows Spencer Olham, a designer of top-secret government weapons. One day while on his way to work, he is arrested by Major Hathaway of the Earth Security Administration (ESA), being identified as a replicant created by the aliens. The ESA intercepted an alien transmission which cryptanalysts decoded as programming Olham's target to be the Chancellor, whom he was scheduled to meet. Such replicants are perfect biological copies of existing humans, complete with transplanted memories, and do not know they are replicants. Each has a powerful "U-bomb" in their chest in the exact design of a human heart, which can only be detected by dissection or a high-tech medical scan, since it only arms itself and detonates when it gets in close proximity to its target. Detection via the special scan works by comparing against a previous scan, if there was one.