When a magical illness begins to afflict children in Boston, the Department of Unorthodox Affairs tasks junior Auditor Grimshaw Griswald Grimsby with uncovering the source and - together with his partner, the grizzled veteran Huntsman Mayflower - putting a stop to it by any means necessary. But their efforts to determine the origin of the spreading malady are soon complicated by the emergence of ancient and strange beings that Grimsby and Mayflower have never encountered before. To uncover the truth, they will have to turn to old acquaintances and allies and delve deep below Boston, into the city of monsters that lies beneath, and travel to the unfathomable reality of the Elsewhere. Burdened by his own troubles, both past and present, it will take all Grimsby's skill, iron, and no shortage of magic to stop this apparent plague and save those who have been afflicted. Assuming it's not already too late.