Rising above the city of Collis is the Holy Santcum. Catrin, an orphan girl, watches over its many spires with a unique skill that serves the Sanctum's master architect. But when Catrin witnesses a murder she is pulled into a dangerous chain of events where the only certainty is that the killer will strike again. The detective on the case is the brilliant and enigmatic Simon, whose insights into the mind of a predator are frighteningly accurate. As the grisly crimes continue, Catrin finds herself caught between killer and detective. But she's hiding her own secret - and it might be the only thing that saves her and those she loves from becoming the next victims . . .