Set in a future where humanity is colonizing planets across the Solar System, the story follows a soldier stationed on Venus. Tasked with recovering a crystal of vital strategic importance, he soon finds himself caught in a trap laid by the planet's native, lizard-like inhabitants — a deadly labyrinth of invisible walls.

The original version of the story, written by the young Kenneth Sterling (based on the idea from Edmond Hamilton's story "The Monster-God of Mamurth"), was half as long. Lovecraft rewrote the work, preserving as many original elements as possible.

The writers adorned "In the Walls of Eryx" with playful references to known individuals in their surroundings (for example, "effjay weeds and slimy ackmanths" are references to the disliked Forrest J Ackerman).