Teenage siblings Lerris and Graika of the planet Soranen want to go into Outer Space, an act forbidden by their people, the Sor. The Sor, it is believed, are the descendants of the Ellarisor and the Humans, now both extinct on Soranen. When Lerris surfs above the clouds in his minijet, he spots a vessel owned by Lorfallin, a chemical magnate. The ship is spraying a green mist that, it turns out, causes a disease which affects those with Human ancestry, including the siblings’ father.
Thus begins a journey to find pure descendants of the Ellarisor, whose blood might provide an antidote to the green mist. Along the way, Lerris and Graika learn the truth about their ancestry, and some truths about Life. This prepares them for the final battle—against the Tusk, who have engaged Lorfallin to destroy all trace of Human life. If they win, Outer Space may be open to them. But if they lose . . .