Zach Adams is one of the best detectives in the country. Nicknamed Cowboy, he's a soft-spoken homicide detective known for his integrity and courage under fire. He serves on a federal task force that has a single mission: to hunt down Dominic Abend, a European gangster who has taken over the American underworld.

In a centuries-old forest under a full moon, a beast assaults Zach, cursing him forever. In the aftermath, he is transformed into something horrible--something deadly.

Now, the good cop has innocent blood on his hands. He has killed--and will kill again--in the form of a beast who can't be controlled or stopped. Before he can free himself, he's going to have to solve the greatest mystery of all: How can you defeat evil when the evil is inside you?