When the body of a young female trainee is found horribly murdered in the Nurses' station of the Western State Mental Hospital, Massachusetts, there is apparently no shortage of suspects - a whole hospital of them. One inmate claims to have seen the killer who he will only describe as The Angel. Twenty years later, Francis Petrel, once a patient at the hospital, writes his account of the events of the murder and its investigation on the walls of his tiny apartment. As he writes he is visited by hallucinations, by the increasing anxiety at his loosening grip on reality. As he goes deeper and deeper into his story about those events, he plunges further into the return of his own madness. He remembers how he is co-opted into the investigation by Lucy Jones, a driven young profiler who has her own reasons for pursuing this particular killer. But she, and Francis, face the same conundrum: how does one find a cold blooded killer masquerading as mad in a world populated by the deranged? In the end it will come down to Francis - he is the only one of the investigating team capable of recognizing the essential lie that The Angel embodies.