Humphrey Sweet is in a secure mental hospital, in the Dangerous and Severe Personality Disorder Unit, after confessing to three child murders. Immune from police prosecution unless declared sane and fit for trial, he is determined to earn respect from fellow inmates and staff and achieves some of this recognition by manipulating them to his will. Home Office psychologist Dr Maurice Simnal is asked to compile a report in conjunction with Dr Ruth Day, the hospital's duty psychologist. He interviews Sweet on a weekly basis, but as he and Day visit family and associates of Sweet, they begin to uncover a trail of further murders pointing to a hitherto unidentified brand of serial killer. As the relationship between Simnal and Day develops from professional to personal, it becomes clear that mass-murderer Humphrey Sweet may be able to reach out from the confines of his hospital room and be threateningly and dangerously sane...