Lady Geiléis, a noblewoman from the northern border, has asked for the prince of Dalriada's help in expelling a howling creature from an old tower on her land - one surrounded by an impenetrable hedge of thorns. Casting a blight over the entire district, and impossible to drive out by ordinary means, the mysterious presence threatens both the safety and the sanity of all who live nearby.

As Blackthorn and Grim begin to put the pieces of the puzzle together, it becomes clear that a powerful adversary is at work.

Their quest is about to become a life and death struggle - where even the closest of friends can find themselves entangled in intrigue.