Bradfield is in panic as someone preys on its most vulnerable - its hospital patients. Carol must juggle how to warn the public and how to keep a predatory MI5 official at bay. The only way out is to solve the case fast - but there is very little to go on, even for Dr Tony Hill.
Carol's investigation into a domestic murder takes a darker twist when the victim, who had been recovering well in hospital, is murdered. As a precaution she 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 suggests the killer may believe he is healing victims in some way, killing for religious belief - but the thought of a compulsory euthanasia killer brings little comfort to the investigative team, or the victims' 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. MI5 get involved when Islamic terror becomes one of the lines of inquiry. Carol has another layer of command that has to be constantly satisfied, and when Brandon falls ill, Carol loses her only political protection. Tony and Carol must follow their instincts, but with Merrick and Kevin competing for Carol's old job of DI, they've got no one but themselves to turn to for help.