The narrative opens twelve years after John Carter's mysterious departure from Earth, resuming his second perilous journey to the dying planet of Barsoom. Seeking his lost princess, Dejah Thoris, Carter materializes not in the familiar red lands of the great waterways, but within a strange, beautiful, and forbidden valley. Here he encounters the grotesque plant men, horrific ten-legged banths, and enormous white apes that guard the sacred river Iss.
Teaming up with Tars Tarkas, the mighty Jeddak of Thark, Carter discovers the horrifying truth behind the ancient Martian pilgrimage: the Valley Dor is no paradise of love and peace, but a slaughterhouse peopled by savage beasts and ruled by the deceitful Holy Therns. These self-proclaimed gods prey upon the deluded believers of the outer world, maintaining a monstrous deception to secure labour and sustenance.
As the narrative develops, Carter and Tars Tarkas navigate labyrinthine corridors of gold and ruby, battling the lesser therns and uncovering secret passages that lead deeper into the planetary crust. They encounter Thuvia, a captive red Martian woman with a mysterious affinity for the ferocious beasts of the underworld, who becomes their invaluable guide through the subterranean perils of the Golden Cliffs.
Escaping the immediate dangers of the valley, the protagonists are captured by the Black Pirates of Barsoom—the First Born—who rule the hidden underground Sea of Omean. Transported to the polar stronghold, Carter confronts Issus, the terrifying old woman who poses as the Goddess of Life Eternal. In a dramatic climax, Carter and a mysterious red youth lead a slave uprising during the bloody rites of Issus, storming the golden throne and shattering the religious hegemony of the First Born.
Throughout these harrowing trials, Carter fights not only for his own survival but to shatter the wicked superstitions binding a world. The narrative weaves intense action with profound realizations about the nature of power, culminating in a breathless escape that leaves Carter poised to bring the truth back to the red deserts of Barsoom and rescue his lost princess.
