As danger increases in the County, Tom is sent far north by his master to be trained by Bill Arkwright, another Spook. Arkwright lives in a haunted mill on the edge of a treacherous marsh and his training methods prove to be harsh and sometimes cruel. But he has toughened up many previous apprentices, and now he must do the same for Tom and prepare him for the gravest dangers of his life. But when the Fiend sends his own daughter, the ancient and powerful water witch Morwena, to destroy Tom, Arkwright makes an error of judgement and Tom finds himself facing his enemies alone.
Realising his danger, the Spook and Alice hasten to his aid. But will their combined strengths suffice in the face of such terrible dark power? And what is the Spook's mistake, the consequences of which might give final victory to the dark?