The local people avoid the place, and there are strange stories of those who dared to venture there either disappearing, or returning insane and inexplicably altered.

The contents of the scroll, covering a large part of the narrative, describe the travels of one Pánfilo de Zamacona y Nuñez, an Asturian explorer, almost 400 years prior.

Zamacona recounts how he was a part of an expedition from Mexico to North America, and how, through the help of a Native American, he discovered a vast underground world filled with grotesque temples, and populated by strange beasts and a highly advanced telepathic civilization who worshipped Cthulhu, Yig, Shub-Niggurath, and—until a certain incident—Tsathoggua.

The underground people also engaged in sadism, depraved practices, ritualistic orgies, and unspeakable horrors such as random body modifications and mutilations of other slave species as entertainment, in order to gratify their time-dulled senses.

As Zamacona observed their decaying social condition and their reactions to his telling them of the surface people, he feared that they would one day decide to invade the outside world, where, given their advanced powers, they would be unstoppable.

T'la-yub was sentenced to unspeakable tortures and mutilations at the amphitheater and ended up as a headless zombie guarding the entrance, while Zamacona was spared because they wished to extract more of his knowledge.

Driven to near-hysterics already, the narrator finally comes across a fully material entity at the sight of which his nerves completely break down, sending him fleeing wildly back to the surface.

That entity is revealed to be the completely mutilated and reanimated corpse of Zamacona with a message inscribed onto his chest in broken Spanish by the underground race.

The outline was so brief it allowed for a great deal of license, so he made it into a 29,560 word story about a mound that conceals a gateway to a subterranean civilization, the realm of K'n-yan, which one of the main characters enters and lives in for a while.

[2][3] The mound in the story is located in Binger in Caddo County, which is a real town about 60 miles (100 km) southwest of Oklahoma City.