Mackie Doyle is a replacement. When his sister was four, a man in a black coat came in through a window and grabbed her baby brother. In his place, a thing was left. Emma walked to the crib and put her hand between the slats. She pulled her hand out but didn't leave when it tried to bite her. They spent the night staring at each other. The thing turns out to be Mackie, her new baby brother.
Mackie lives in the town of Gentry, a former steel town. During the day, the town is a "sea of minivans and lawns and golden retrievers." Almost everyone works at one of the computer plants, the dairy farm or the community college. Gentry is the town where everyone wants to be and people rarely leave it's prosperous borders.. But at night, it's a different story. The town is full of secrets. Residents double lock their doors and follow superstitions in an attempt to keep the unwanted out of their lives.
The Doyle family works hard to keep Mackie's secret. It is never discussed and his exposure to metal is kept to a minimum. His pastor father has trained him from an early age not to draw attention to himself so that no one will notice that something is not right about Mackie. But when Tate Stewart's baby sister dies, she refuses to stay quiet about what is going on in the town. She comes to Mackie with her questions and accusations and refuses to take no for an answer. Unfortunately for Mackie, he begins to realizes that living with humans for sixteen years is a rarity. He is slowly dying from his constant exposure to metal. Soon Mackie discovers the secret world that exists under the town of Gentry, the world which he is from. When he is offered a cure for what ails him, Mackie must decide if feeling well is worth the cost, as Tate continues to push him to see that things aren't right in Gentry.