Alien Citadel takes place on Earth, but on a future Earth that only barely resembles our own world. In this future world, humans live in fear, hiding from the Slavers, the planet's new masters. The planet is covered by desert wasteland, forested wilderness, and radioactive firesands where monstrous mutant creatures prowl. There is no more civilization and little hope. The Citadel, built underground in a mountain hideaway, is where the Slavers take those humans whom they spare for use as slave labor: child-bearingage women are used for breeding experiments, and the rest of the captives are used as laborers in the Slavers' mines.

As the novel opens, Finn Ferral has just travelled across a continent strewn with dangers. Slavers run rampant over the planet, scouring the land with heat weapons in their efforts to eliminate any humans and other "vermin" still infesting its farthest reaches. But the Slavers...