Jim Brill ventures into the Gobi Desert, searching for Richard Barlow, a scientist who disappeared four years prior. Brill undertakes this dangerous quest at the request of Gloria Barlow, Richard’s wife, whom Brill secretly loves. Gloria had not been happy with her brutal husband but felt obligated to attempt his rescue.

Brill's expedition faces disaster when it is attacked by Mongol bandits, and he is the only one to escape. Struggling to survive on a tired horse amidst a sandstorm, Brill presses on toward a wild and rocky range of hills, pursued by the Mongols. Just as they close in, a gigantic spider appears and attacks the Mongols. Brill manages to shoot the spider and continues onward, exhausted and nearing collapse. He eventually stumbles upon a strange city hidden in the hills, occupied by the degenerate descendants of Genghis Khan’s warriors. To his shock, Richard Barlow has become the ruler of the city and lives with a Chinese woman he had captured from a caravan.

Barlow reveals that he has created monstrous, giant spiders to protect the city, transforming small spiders from local caves into the fearsome creatures through his scientific experiments. He shows Brill a meditation chamber once used by Genghis Khan and shares his plan to invoke the conqueror’s ghost. Barlow believes that thoughts leave an imprint on material substances, much like physical objects, and that he can develop these impressions, much like photographic negatives, to gain the power of Genghis Khan’s mind and spirit. Barlow’s own transformation into a Mongol has already begun, as he shows Mongolian characteristics and speaks with a guttural accent.

When Brill mentions Gloria, Barlow sneers and declares that she should be brought to him. Brill refuses, leading to a brutal fight between the two men. Though Brill knocks Barlow unconscious, he is eventually overpowered by Barlow’s servants. However, the Chinese woman, who overheard Barlow's demand for Gloria, stabs him to death. At the same time, the city is raided by a band of Mongols led by an old friend of Brill’s. The Mongols free Brill, and his friend escorts him back to civilization.