When the body of a young female nurse is found horribly murdered in 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 goes deeper and deeper into his story, 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 for he is the only one of the investigating team capable of recognizing the essential lie that The Angel embodies: for though his acts are those of a mad man, he most emphatically is not...