Twin Planets—Earth and Firma were mirror worlds. Because they were on a single time-track, Earth was destined to repeat the dangerous history of Firma—which had been stopped in its rotation around the sun by Aliens. Denning and Liston were twin humans, biogenetic supermen with a single mission: to get Firma moving on its axis again and destroy its Alien conquerors. Though Firma was Liston's universe, Denning had to aid him, or Earth, too, would one day be burned to a cinder by the heat of the sun.