Liesl lives in a tiny attic bedroom, locked away by her cruel stepmother. Her only friends are the shadows and the mice—until one night a ghost appears from the darkness. It is Po, who comes from the Other Side. Both Liesl and Po are lonely, but together they are less alone.

That same night, an alchemist's apprentice, Will, bungles an important delivery. He accidentally switches a box containing the most powerful magic in the world with one containing something decidedly less remarkable

Will's mistake has tremendous consequences for Liesl and Po, and it draws the three of them together on an extraordinary journey.