Jinx is a captive of Holly. Izzy is a distant dream as he is continually tortured and abused. She travels back to the last two places where Crom''s siblings are entombed and frees them. She uses their magic to transform herself, killing them as Crom taught her to do. She plans to sacrifice Jinx to raise them. But the proto-demons are more dangerous than she could have suspected (or Crom set her up to free them) and they trick her, taking possession of her instead. She becomes Cuilieann, an older more dangerous form of Holly, part fae, part angel, part god. She drags Jinx with her, all set to ravage the earth.
Izzy is missing, walking the Sidhe ways with Amadán who is using her to shore up support among the Aes Sidhe. She begins to notice a degradation of the Sidhe ways. They are breaking down, spilling out into the human world. She talks to her father who begs her to come home. Brí is trying to help Silver and Dylan, while at the same time comforting Izzy''s distraught parents. She is bewildered to find everyone turning to her. Blythe on the other hand is enraged. The Cú Sidhe pack searches everywhere for Jinx.
He was free, he was one of them, and Holly took him. The hounds vow no mercy. War is looming within the ranks of the Sidhe. Silver is doing all she can to keep the peace, but is aware that her power is starting to wane. She needs to give Dylan the kiss of the Leanan Sidhe again, which she fears will tumble him into madness and destroy him, ultimately killing him. With Brí agreeing to help her, for Izzy''s sake, she finds that since he was poisoned by one of the Croms, the venom of the others might save him. Cuilieann is an unstoppable force. Silver tries to hold back the corruption taking place in the Sidhe ways and Brí searches for a cure.
The angels and the demons begin to draw lines, daring each other to transgress and start another war. Amadán meanwhile brings Izzy to a secluded island where the harp of the Dagda Uaithne is held, chained to a rock. Its music is enchanting and she almost is bespelled by it. Jinx is there. Izzy can''t believe it, but it''s really him, hurt and wounded, almost broken. Izzy is so relieved to see him alive, the spell slides off her. Izzy''s father goes in search of her, finding her there and fighting the Magpies to reach her. Amadán offers David a truce.
He tells him about Holly and what she has done. David and Izzy don''t trust him (and never have). The harp, he says, is the only way to stop what Holly has become. Holly and Amadán are in collusion. The harp is the one thing that can stop her and they couldn''t touch it before. Now Izzy has it, they can take it from her and destroy it. David is trapped and Izzy at their mercy. Jinx helps her escape.
Reunited, they can''t help but see the feelings they had for each other haven''t gone anywhere. She soothes and heals him, and Jinx is fierce in his protection of her. Exchanging David for Izzy wasn''t Holly''s plan but she decides she can use him all the same. Izzy''s mother and grandmother arrive in Brí''s hollow, determined to free him. Reunited with her family, Izzy is immediately placed under protective custody by Brí. The Hollow is the last safe place, the hill under siege from the dark forces Holly has raised. But Amadán betrays Holly, calling down the angels and summoning up the demons. Its retribution, he says, for all the betrayals she has ever visited on him and his own.
Especially the swan maiden he loved. Izzy frees her father while Amadán stabs Holly and she kills him. Izzy takes the harp back to Silver and when Dylan plays it, the spell binding him to Silver is broken. It empowers her beyond anything he could manage. Silver is able to drive back the monsterous Holly, destroying her. In the peaceful aftermath, the world of the Sidhe is shattered, but when Dylan plays again, the Sidheways reform themselves. Amergin and his sorcerers were human, Brí reminds them, and when it comes to that harp, they have more power than all the Sidhe together. Luckily some of them use it wisely.
She leaves it in his hands, the safest hands of all.