A group of patients who are all on the same ward at a local hospital die within minutes of each other. When the cause of death is found to be an overdose of drugs, and each of the victims has the tattoo 'let me go' stencilled onto their back, Creegan and Taylor investigate the hospital cleaner, Carl Burgess, who is found in possession of items belonging to a senior doctor. Creegan is left unconscious after being attacked at home, but who is responsible?

The investigation's attention turns towards locum Julie Carney following the attack on Creegan. When she commits another murder in Wales, Creegan realises that he needs to set a trap to lure her home. Rivers and Krietman's pursuit of an unstable Carl Burgess results in Burgess trying to kill a patient, and then electrocuting himself, from which he dies of his injuries. When Julie Carney resurfaces, Creegan is forced to watch on as she tries to commit suicide.