Somewhere, from among the four human-settled worlds of the Warden Diamond, hostile aliens were spying on Earth. But no agent could be sent to investigate and report back; a symbiont invaded all life forms and destroyed any form of machinery. That called for extraordinary means. One agent was chosen, then four men were stripped of their own minds and personalities, and his was imposed upon them. hooked up properly, he could then receive their reports, without ever leaving safe territory. Each man was assigned one world to conquer. His mission was first to find the Overlord of that world and kill him, then to take over his link with the aliens. Of course all this must be done with no help beyond his own naked ability.