Thomas is an ordinary, thirteen-year-old boy who thinks his eyes are playing tricks on him the night that he sees his baby sister Isabelle being taken by two strange creatures. The next day his worst fears are realized. He was not dreaming. His sister is gone and a changeling has been left in her place. Now Thomas and his nerdy, know-it-all sister Libby must embark upon the adventure of a lifetime to the magical world of Faylinn, where they will enlist the help of a sidhe, a fae, and even mermaids to fend off evil faeries of all kinds, including a mighty peist. But as it turns out, their biggest enemy of all is the one that they trusted the most.