Jim Brill, the lone survivor of a desert expedition, flees from Mongol bandits across the Gobi Desert. His guides and servants have been slaughtered, leaving him to escape the assault amidst a sandstorm. Struggling to survive with limited supplies and a weary horse, Brill heads toward a desolate mountain range where no white man has ventured and lived to tell the tale.

As he enters a canyon, Brill is pursued by the bandits. He hides and readies his revolver for a last stand but witnesses something extraordinary: a massive, monstrous spider emerges from the rocks, attacking the Mongols. Though horrified, Brill takes advantage of the confusion to kill the creature before more giant spiders swarm into the canyon. He narrowly escapes up the mountains, driven by his need to find a man—Richard Barlow—who he both hates and seeks for personal reasons.

Brill’s journey brings him to the city of Khor, hidden deep in the mountains. Built by Genghis Khan and long forgotten by the world, the city thrives under the rule of its mysterious inhabitants. To his shock, Brill finds that Barlow is not only alive but has become the ruler of Khor. Once a scientist, Barlow now claims the title of Ak Khan, the "White King," ruling the people through advanced technology and fear.

Barlow explains how he used his scientific knowledge to convince the superstitious inhabitants of Khor of his mystical powers. He employs electricity and other inventions to maintain control and protect the city from Mongol raiders. He also reveals his experiments on human subjects, boasting of his progress in manipulating life forms. The giant spiders, for example, are one of his creations, developed to terrorize the Mongols.

Barlow’s true obsession, however, lies in psychic experiments. He claims to have discovered how to absorb the thoughts and essence of Genghis Khan from the walls of a meditation chamber within the palace. These psychic impressions, according to Barlow, allow him to acquire the conqueror's genius and ambitions. Barlow's personality is slowly transforming into that of the ancient Mongol emperor, as he prepares to dominate Asia.

Brill, horrified by Barlow’s descent into madness, confronts him. The tension between the two men escalates, especially when Barlow insists that Brill bring his wife, Gloria, to Khor as part of his plans. Barlow intends to offer Gloria as a gift to Togrukh Khan, the leader of the Mongols, solidifying his alliance with the nomadic tribes. Enraged, Brill attacks Barlow in a brutal fistfight, nearly killing him before Barlow’s servants intervene and knock him unconscious.

Brill awakens bound but is soon freed by Lala Tzu, Barlow’s Chinese concubine, who has grown jealous and fearful of the scientist’s obsession with Gloria. Lala Tzu stabs Barlow, killing him just as his transformation into a Mongol reaches its climax. Brill, realizing the danger they both face, follows Lala Tzu as they flee through the palace.

With the Mongol forces attacking Khor, Brill and Lala Tzu reach the palace’s dome, where Barlow’s ultimate weapon—a machine capable of emitting a deadly ray—has been set up. Brill uses the weapon to annihilate the Mongols at the city gate, saving the inhabitants of Khor. In gratitude, the people of Khor offer Brill the throne once held by Barlow, their "White King."