Mary Catherine 'MC' Riggio and Kitt Lundgren, detective partners at the Rockford, Violent Crimes Bureau, have been through a lot together. Lately, though, things have begun to look up: MC is about to get married, while Kitt is slowly coming to terms with the loss of her daughter.

Suddenly, the fragile peace of their world is shattered when MC's cousin Tommy is found brutally murdered at his home. Suddenly, nothing will be the same again. A killer is on the loose, someone who quickly and systematically works his way down a seemingly unconnected list of victims. With the body count rising, MC realises that she has to keep her head in the game and use her grief to fuel the hunt, but as the madman draws ever closer, Kitt and MC begin to struggle to walk that fine line between protecting the law - and taking it into their own hands ...