The Farren family has been a plague upon Philadelphia's most dangerous neighbourhood for generations. There, row after row of tumbledown houses hide dark secrets - none darker than Billy, the youngest Farren. Afflicted by a syndrome that means he can't recognise faces, he uses photographs to identify his family - and his victims. And when their life has bled away, he takes a final, gruesome picture for his wall. But what is the meaning of the horrific ritual he enacts with every murder? And is there any connection to a childhood event Detective Kevin Byrne has buried so well it's hidden even from his former partner?