In the story, a detective, Erik Lönnrot, attempts to solve a mysterious series of murders which seem to follow a kabbalistic pattern.
By following what seem to be clues, the detective falls victim to his belief in abstract reason and to the man whom he presumes to be a criminal mastermind.
Based on a cryptic message left on the rabbi's typewriter—"The first letter of the name has been uttered"—the detective determines that the murder was not accidental.
In the meantime, the detective's office receives an anonymous tip to view the locations of the murders on a map, revealing that each coincides to the point of an equilateral triangle.
Recognizing that the southern end of the city has yet to be terrorized, Lönnrot extrapolates that the complete pattern will create a rhombus (the south appears frequently in Borges's writings as an allusion to the Argentine frontier, and by extension, as a symbol of solitude, lawlessness, and fate).