Young Tom, the seventh son of a seventh son, starts work as an apprentice for the village spook, whose job is to protect ordinary folk from "ghouls, boggarts, and all manner of wicked beasties." Can Thomas succeed? Will he learn the difference between a benign witch and a malevolent one? And what will happen if Thomas accidentally frees Mother Malkin, the most evil witch in the county?

Apprentice Tom and the Spook for whom he works flee to Ireland, where a powerful witch has joined forces with the dark mages trying to call forth ancient powers to destroy Tom.

As creatures of the dark hunt for the witch assassin Grimalkin, who carries the captured Fiend's head, Spook's apprentice Tom tries to find a way to finish this terrifying evil once and for all.