When DI David Murphy and DS Laura Rossi are called to investigate the murder of a student at the City of Liverpool University, they find a letter from her killer attached to her body detailing a famous and deadly unethical psychological experiment that had been performed on her. Convinced at first that the murderer is someone close 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. Meanwhile 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 firmly pointed at him. And as the two seemingly unconnected events collide, Murphy and Rossi realise they are chasing a killer unlike any they've faced before...