Destroyed by fire years ago, the infamous Philadelphia State Hospital was known as a warehouse for the criminally insane.

But one man never left.

By night Luther walks Philadelphia's backstreets, drawing to him the mad, the corrupt, the fallen. By day he roams the catacombs beneath the city, killing his prey.

When Detectives Kevin Byrne and Jessica Balzano are called to a bizarre murder scene: a man has been killed by a railroad spike driven into his head and left sitting on a bench in a local park, it is just the beginning of a trail of evil that leads back to the hospital and the nightmares it still contains...