When sixteen year-old Brenna Morgan arrives in Ireland with her travel-writer mother, she expects the usual—a few months in yet another country that isn't home. What she doesn't anticipate is running into a dying fairie who saves her life in exchange for a promise to protect a child hidden in the fairie realm. Armed with only her wits and a strange iron key given to her by the fairie, Brenna is pulled into a world where myth and legend cross all too often into reality.

She is aided in her search by her newfound friend Patrick, and a reluctant fairie with a grudge against humans. But the more she uncovers the more she starts to realize that not all is as it seems, that danger comes in the most unassuming of guises, and that the child she swore to protect could destroy not only the fairie world, but her own as well.