The United States government is given a warning by the preeminent biophysicists in the country: current sterilization procedures applied to returning space probes may be inadequate to guarantee uncontaminated re-entry to the atmosphere. Two years later, seventeen satellites are sent into the outer fringes of space to "collect organisms and dust for study." One of them falls to earth, landing in a desolate area of Arizona. Twelve miles from the landing...

In the Nevada desert, an experiment has gone horribly wrong. A cloud of nanoparticles--micro-robots--has escaped from the laboratory. This cloud is self-sustaining and self-reproducing. It is intelligent and learns from experience. For all practical purposes, it is alive. It has been programmed as a predator. It is evolving swiftly, becoming more deadly with each passing hour. Every attempt to destroy it has failed. And we are the prey.