Seven-year-old Henry Day is kidnapped by fairy changelings living in the dark forest near his home - ageless beings whose secret community is threatened by encroaching modern life. They give Henry a new name, Aniday, and the gift of agelessness - now and forever, he will be seven years old.

The group has left another child in Henry's place. This changeling boy, who has morphed himself into Henry's duplicate, must adjust to a new way of life and hide his true identity from the Day family. But he can't hide his extraordinary talent for the piano, and his near-perfect performances prompt his father to suspect that he is an impostor.

As he grows older the new Henry Day becomes haunted by vague but persistent memories of life in another time and place. Both Henry and Aniday search obsessively for who they were before they changed places in the world.