Conan and his ally, Natala the Brythunian, are the sole survivors of Prince Almuric's army which swept through the Lands of Shem and the wilderness of Stygia. With a Stygian host on their heels, Almuric's soldiers had cut their way across the kingdom of Kush, only to be annihilated near the edge of Stygia's southern desert.

In the resulting conflict, when the Stygians and Kushites surrounded the trapped remnants, Conan sliced his way through the Stygian militia and fled on a camel, with Natala, into the southern desert. For days, the two pushed on, seeking water, until their camel died. Then, they continued on foot.

When their canteen is empty, Conan prepares to slay Natala in an act of mercy-killing. However, they spy on the distant city of Xuthal. Eventually, Conan and Natala enter Xuthal while pursued by an entrance guard. They soon encounter Thalis, a beautiful Stygian mage, who reveals the history of her fabled city and the existence of Thog. Thog is a monstrous demon from the city-states of ancient Valusia, his current form summoned by the sorcerers of Xuthal from the darkness between the stars. For an ageless time, Thog has haunted the depths of Xuthal in search of living flesh to assuage the continuing manifestation of his body on the physical plane.

Thalis fall in love with Conan and, to eliminate her rival, kidnaps Natala in the hopes of sacrificing her to Thog. However, Thalis pauses to strip Natala of her tunic and, with a jewel-handled whip, flagellates the Brythunian. In the middle of this bizarre act, Thog suddenly appears, snatches Thalis, and devours her. The demon returns for Natala, but Conan intervenes and saves her. Conan fights Thog with all his might but is scarcely harming the demon's supernatural form while receiving hideous wounds in the coils of its pseudopods and tentacles. However, Conan manages to pierce what he perceives as the "head" of the monster from below and precipitates it down a well. Conan frees Natala, who sets forth to help him, but he's rapidly dying (this is the closest Conan ever gets to death in all of his sagas). Fortunately, the Brythunian girl soon brings him a jade goblet full of golden wine, retrieved from a room with a dreaming woman of Xuthal in it. The beverage proves to be a life-giving elixir briefly mentioned by Thalis in a previous conversation, which miraculously heals all of Conan's wounds. Finally, the couple retrieves enough food and water to cross the rest of the desert. The two depart toward the horizon while Natala jokingly blames Conan for having aroused Thalis' lustful nature and him retorting playfully about women's jealousy.