In the small town of Binger, Oklahoma, there lies a mysterious mound that has captured the curiosity and fear of the locals. Tales of haunting by a Native American man during the day and a headless woman at night have circulated, warning people to stay away. Intrigued by these stories, an ethnologist decides to investigate the mound in 1928.

At first skeptical, the narrator equips themselves with archaeological tools and ventures to the mound. Upon arrival, they notice a man pacing the area who seems to resemble Native Americans, but cannot be linked to any known tribe. However, it is not until the narrator is given a talisman made of an unidentified metal by a local chieftain that they make a startling discovery.

Unearthing a strange cylinder made of the same mysterious metal, the narrator finds it covered in hideous engravings and strange hieroglyphics. The excitement grows as they translate a scroll written in Spanish, found inside the cylinder. The scroll reveals an incredible account of the travels of Pánfilo de Zamacona y Nuñez, an Asturian explorer who journeyed to North America nearly 400 years before.

According to Zamacona's narrative, he stumbled upon a vast underground world during his expedition. This underground realm was filled with grotesque temples and inhabited by a highly advanced telepathic civilization. The inhabitants worshipped various deities, including Cthulhu and Yig, but had recently abandoned the worship of Tsathoggua due to a horrifying discovery.

It is revealed that the underground race, known as the K'nyanians, had achieved immortality and possessed technology to modify other races and reanimate the dead as slaves. However, their civilization was plagued by sadism, depraved practices, and unspeakable horrors. Desperate for stimulation, the K'nyanians welcomed Zamacona as a visitor from the outer world.

As Zamacona learned more about the K'nyanians, he became increasingly fearful of their advanced powers and the potential danger they posed to the surface world. However, he was trapped underground by his hosts, who feared that his knowledge would attract an invasion of Europeans driven by their insatiable greed for gold.

Zamacona's attempts to escape with a K'nyanian native ended in betrayal and capture. T'la-yub, the female K'nyanian, was subjected to unspeakable torture, while Zamacona was spared due to his knowledge. In a desperate last attempt, Zamacona managed to deposit the scroll-containing cylinder on the mound before his demise.

The narrator, still skeptical but intrigued, returns to the mound the next day. They discover a staircase leading deep underground and encounter dematerialized beings patrolling the tunnel. Protected by the talisman, the narrator stumbles upon the remains of previous explorers and their equipment that have partially dematerialized.

As their nerves reach a breaking point, the narrator finally comes face to face with a fully material entity. It is the mutilated and reanimated corpse of Zamacona, bearing a message inscribed on his chest in broken Spanish. The chilling message reads, "Seized by the will of K’n-yan in the headless body of T’la-yub."

The narrator's skepticism is shattered, and they flee in terror from the mound, leaving behind a world of unimaginable horrors buried beneath the surface. The haunting tales and strange occurrences in Binger, Oklahoma, are no longer mere legends but a terrifying reality that will forever be etched in their memory.