With a werewolf and a vampire for company, Lacey Swift makes her way across a vast wasteland in search of the prisoner who has the key to the box, which contains a shard of broken mirror - a sliver of mirror that will restore peace to the world she now finds herself in.
As they pass through the Mirror Realm, Lacey and her friends stumble across a dying man. He tells them that if they want to reach the box, they will have to cross a desolate land known as the Emptiness - a place where the dead roam and from where no one has ever returned.
Unbeknown to Lacey, her identical twin sister, Thea Swift, has been snatched into the Mirror Realm and is being led to her death. With time running out, Lacey and her friends go in search of an ancient and mythical steam train, named the Scorpion Steam, which they believe will carry them safely across the Emptiness and to the broken piece of mirror they are searching for.
As Lacey makes her journey, her enemies are one step ahead of her and growing ever closer to murdering her twin sister and destroying the world she has promised to save.