DI David Murphy and DC Laura Rossi investigate the murder of a student at the City of Liverpool University. Attached to her, is a letter from her killer which details a famous unethical psychological experiment performed on the victim, resulting in her death. Convinced at first that the murderer is someone close and known to the victim, Murphy dismisses the letter as a bid to throw them off the scent, until more bodies are found, each with their own letter attached. On the other side of the city, Rob Barker, an admin worker at the university, is dealing with his own loss. His partner has been missing for almost a year, with suspicion from all around her firmly pointed at him. As the two seemingly unconnected events collide, it becomes apparent Murphy is chasing a killer who sees his victims as collateral damage - someone who kills to discover more about life.