John Cain has been working Homicide long enough to get hard to pretty much every wrenching personal loss. Today, it gets too much. He gets assigned something that finally breaks through his defenses -- a child killing that hits close to home. Until now, he's been a cop who goes by procedure because it's the best way to ensure that scum go to prison. Today, he makes the parents of the victim a chilling promise. He will find the killer using any means necessary. How much of a monster do you have to become to hunt monsters?