Walter Gilman is a Miskatonic student of mathematics and folklore who takes lodgings in the Arkham garret once occupied by the seventeenth-century witch Keziah Mason. In recurring dreams he traverses geometric spaces beyond human perception.
Keziah Mason and her rat-like familiar Brown Jenkin intrude on his dreams and seek to draw him into a sacrificial rite. Gilman dies on Walpurgis Night trying to prevent a child's sacrifice.