It is the autumn of 1160, and twelve-year-old Agnes is helping with the harvest when she hears a frightened voice calling from the nearby woods. When she goes to investigate, Agnes can't believe what she sees. There, at the bottom of the deep wolf traps, are two children. They are shouting in a language no one understands--and their skin is green.

Agnes soon discovers that these are no ordinary children; in fact, they aren't even human. They are of the Fair Folk, and they are here to take Agnes home to their world. Trusting that the Fair Folk cannot lie, Agnes agrees to venture underground. But she soon learns just how dangerous their world is--and what it will take to break the ancient bargain meant to keep her there.