Earth and Firma were twin planets - mirror worlds in a single time-track. Now Firma was halted in its rotation around the sun by the Aliens. Unless Denning and Liston, twin humans, could destroy the Aliens and get Firma moving again, Earth would some day repeat Firma's tragedy and be burned to a cinder.
The Aliens had an incredible array of weapons at their disposal.
Denning and Liston had only their courage and their brains.