Carol’s investigation into a domestic murder takes a darker twist when the victim, who had been recovering well in hospital, is murdered. The crime has graduated from being something that happened in the heat of the moment to something with far more premeditation. As a precaution, Carol orders post mortems of other patients who had died in that hospital of “natural causes”, and finds out that she has a lot more murder on her hands than she’d bargained for. Tony begins to establish that this killer may believe they are healing their victims in some way killing for religious belief – but the thought of some form of compulsory euthanasia killer brings little comfort to the investigative team, or the victim’s families. Tony and Carol begin to unearth the undercurrents and tensions that lie within the hospital structure, but that only yields more suspects, rather than narrowing the search. The investigation suffers a further thread when MI5 get involved, as Islamic terror becomes one of the lines of inquiry. Carol finds herself with another layer of command above her that has to be constantly satisfied, and when Brandon finds himself in need of medical attention, Carol loses her only political protection. Tony and Carol are forced to follow their instincts, but with Merrick and Kevin concentrating on who will succeed Carol’s old job of DI, they’ve got no one but themselves to turn to for help.