American adventurers Colin “Boots” O’Hara and Archer Kennedy are searching for gold in the wastes of Mexico when they chance upon the plantation of Svend Biornson, his wife Astrid, and their young daughter. The house contains artifacts that the greedy Kennedy recognizes as valuable relics related to the worship of Quetzalcoatl, the ancient Aztec lord of the air. That evening, O’Hara and Kennedy are abducted from the plantation by a band of white Indians and spirited away to Tlapallan, the fabled lost Aztec city whose inhabitants are divided in their worship of the benevolent Quetzalcoatl and the evil Nacoc-Yaotl. The two men escape but are recaptured after causing considerable mischief. Biornson, whose wife is a Tlapallan, intercedes on O’Hara’s behalf and engineers his escape into the desert. Kennedy, who witnessed a sacred ritual of the priests of Nacoc-Yaotl, is left to their mercy.
Fifteen years later, O’Hara visits his sister Cliona and her husband Anthony Rhodes at their bungalow in suburban Carpentier. The house is ransacked three times. After the third incident, O’Hara pursues the apelike creature responsible for the damage, tracking it to an estate in nearby Undine owned by self-styled animal breeder Chester Reed and his daughter.
Bothered by the unwholesomeness of Reed’s business, O’Hara takes Reed’s daughter to stay with Cliona. Upon his return to the estate, O’Hara discovers that Reed is none other than Archer Kennedy, who befriended an ambitious priest of Nacoc-Yaotl during his captivity and learned secrets that helped him to bring about the destruction of Tlapallan. His “daughter” is actually the kidnapped child of Svend Biornson, and his menagerie of animals is the product of experiments with a ritual of Nacoc-Yaotl that allows the scientific dissolution and reconstruction of an organism in whatever form the experimenter wills. The megalomaniacal Kennedy plans to terrorize the world with his godlike power and intends to make O’Hara his first victim. O’Hara is anesthetized before the experiment, and in a delirium he imagines that Kennedy is the avatar of Nacoc-Yaotl and that Biornson’s daughter, who has returned to rescue him, is the reincarnation of Quetzalcoatl. O’Hara is saved when a fire, possibly of divine origin, breaks out in the laboratory, destroying Kennedy and his house of horrors.