The stories of City of Saints and Madmen are set in Ambergris, an urban sprawl named for "the most secret and valued part of the whale" and populated by humans after its original inhabitants—a race of mushroom-like humanoids known as "gray caps"—were violently driven underground. These creatures, though removed from the eccentricities of daily life in Ambergris, continue to cast a shadow over the city with their unexplained nocturnal activities.