In Cherry Creek, Colorado - a suburb of Denver - middle class families are being murdered in their homes. The houses have been robbed only of small items of monetary worth. The victims have all thus far been traditional families - mother, father and children. The parents are gagged, bound and eventually brutally beaten whereas the children are murdered by an injection of phenobarbital. The nature of the deaths lead the BAU to believe there are two unsubs, a dominant and a submissive, the latter who acts as the angel of mercy in the way he kills the children. Through evidence, the BAU know the unsubs work their way into their victims' houses through a trusted ruse of some sort, and after entry the victims have no way of outside contact since phone lines are cut and cell phone access is blocked. They get a break in the case when in the latest attack, there is one survivor, the teen-aged daughter, Carrie Ortiz. Carrie witnessed the murders and saw the unsubs' faces. Other information Carrie provides is that there are indeed two, a dominant and a submissive, who referred to each other as brothers despite being from different ethnic backgrounds. The BAU also believe that the submissive may have had a reason for not killing Carrie since in this latest attack, they took an item personal to that family. When the BAU believe they've identified the unsubs and the rationale for the murders, they realize they have more to worry about than just finding the unsubs. There are also children in potential harm and an environment unwittingly breeding future killers.