Disillusioned healer Blackthorn and her companion, Grim, have settled in Dalriada to wait out the seven years of Blackthorn's bond to her fey mentor, hoping to avoid any dire challenges. But trouble has a way of seeking them out.

A noblewoman asks for the prince of Dalriada's help in expelling a creature who threatens the safety and sanity of all who live nearby from an old tower on her land--one surrounded by an impenetrable hedge of thorns. With no ready solutions to offer, the prince consults Blackthorn and Grim.

As Blackthorn and Grim put the pieces of this puzzle together, it's apparent that a powerful adversary is working behind the scenes. Their quest soon becomes a life-and-death struggle--a conflict in which even the closest of friends can find themselves on opposite sides.