The story, told over three 60-minute episodes, begins when a wave of violence hits London following the discovery of the bizarrely mutilated body of a man in the Thames. First appearances suggest a witchcraft killing. But the dark reality is more complicated – and even more frightening. Assigned to the case is young, hopeful DS Dan Twentyman (Matt Smith) and his senior partner DI Moses Jones (Shaun Parkes), seconded from Scotland Yard due to cultural links with the local community.