The 24th century: humankind has become a spacefaring civilization, colonizing the solar system and beyond. While no alien forms of life have yet been encountered in this expansion into space, colonists suddenly encounter machines of alien origin - huge robots able to reproduce themselves. Called replicators by the colonists, they seem to have but a single goal: to destroy all organic life they come in contact with.

Since the colonial governments have no means to fight this menace directly, they instead promise huge rewards to whoever destroys a replicator. As a result, the frontier attracts a new kind of adventurers, the Hunters, who work to find and destroy the replicators. Mike Edwards, a skilled young maintenance technician and robotics expert at a faraway outpost, will not only become one of them - but be the very first one to unlock the secret behind the replicators' origin and mission.

The scientific and technical aspects underlying the plot - in particular space travel, robotics and self-replicating spacecraft - are introduced and discussed by the author in an extensive non-technical appendix.