The narrative begins in Chicago, where Robert Grandon, weary of an idle life among the wealthy, is abducted by a mysterious doctor who has mastered astral projection and interplanetary communication. Through this esoteric technique, Grandon's consciousness is transferred to the body of a prince who has been enslaved in the stone quarries of Venus by the ruthless Amazon princess Vernia. Awakening in this harsh, humid world, Grandon faces the brutal reality of quarry labor and escapes into the dense, menacing fern forests of the mountainous kingdom of Uxpo. In the wilderness, he encounters the Fighting Traveks, a band of insurgent mountaineers determined to throw off the imperial yoke of Reabon. By defeating their leader in single combat, Grandon assumes command and adopts the persona of Grandon of Terra, wielding an unusual straight-bladed sword provided by an enigmatic ally from the planet Olba.
As the story develops, Grandon finds himself repeatedly clashing with the military forces of Princess Vernia, who personally leads campaigns to crush the rebellion. During a violent forest storm, Grandon rescues a stranded combatant who turns out to be Vernia herself, hiding her identity initially. Their survival journey takes them across treacherous rivers and into the subterranean domain of the flying grampites, winged bat-like monsters that guard vast underwater gardens. Following a fierce battle in the volcanic crater where the grampites nest, Grandon and Vernia escape using an advanced Olban airship. However, a navigation error crashes the craft in a strange valley dominated by colossal trees and intellectual ant-like sabits who domesticate humans as forced laborers.
In this hidden valley, Grandon is captured by the sabits and rises through their brutal hierarchy to become a supervisor, all while planning a massive insurrection. Utilizing ancient Albine armor and weapons discovered in long-forgotten underground armories, he organizes the enslaved humans and stages a successful revolt that breaks the power of the sabit castes. During these conflicts, Vernia is captured by a renegade slave named Tholto, leading to a tense pursuit across underground waterways and salt marshes. Grandon and his loyal marsh-men track the fugitive to a coastal camp, where they discover that Vernia has mysteriously vanished into the jungle.
Leaving the newly founded nation of Granterra under the kingship of his ally Joto, Grandon resolves to return to Uxpo to lead its people to final freedom. Meanwhile, Vernia is recaptured and delivered to her ambitious cousin Destho in Reabon, who uses her disappearance to seize the throne and attempts to force her into marriage. The narrative converges toward a final confrontation between the newly crowned King of Uxpo and the usurping forces of Reabon, setting the stage for a climactic struggle over the fate of two worlds.
