Seven-year-old Henry Day is kidnapped and renamed "Aniday" by changelings, ageless beings who inhabit the woods near his home. The changelings also leave behind one of their own, who flawlessly impersonates Henry except for one noteworthy detail--the new Henry is a prodigiously talented pianist. Both Aniday and Henry settle comfortably enough into their new existences, but both are haunted by vague memories of their former lives.