It's not every day that the Devil knocks on your door. There are a million stories in the world. Most are perfectly ordinary. This one... isn't.

Hannah Green actually thinks her story is more mundane than most. But she's about to discover that the shadows in her life have been hiding a world where nothing is as it seems: that there's an ancient and secret machine that converts evil deeds into energy, that some mushrooms can talk — and that her grandfather has been friends with the Devil for over a hundred and fifty years, and now they need her help.