Carl is a herder, one of the elites who follow their horses through the galaxy in search of new planets. During a passage, Carl's worst nightmare comes true when he falls off his horse. He ends up on Planet Earth, lost for nine centuries and now nearly empty because of a deadly virus.

He's rescued by Ruby and her father, who nurse him back to heath, because Carl isn't immune and nearly dies when he catches the virus. He tries to adapt to "Mother Earth", a planet run by robots, where everyone is either a breeder or a sterile worker. As time passes, he becomes more attached to Ruby. He doesn't know if he can leave Ruby to her fate, or if she'd consider leaving her family and everything she knows to follow him.