Dr. Anna Kane has invented a tiny computer device that, once implanted in the brain of a psychopath, monitors his violent impulses and renders him unconscious before he can act. The device is first implanted in John Chester Marlon, the infamous, charismatic Subway Killer. Convicted of murdering four young women in the tunnels of New York City's subway system, Marlon is so dangerous that even hardened prison guards will not be alone with him. If Dr. Kane's little computer can control Marlon, she'll become rich and famous. But John Chester Marlon is not a psychopath, not subject to murderous rages. His every act is cold, calculated, and drawn from a superior understanding of human behavior. The implant won't put Marlon to sleep. It won't even slow him down. Dr. Anna Kane is about to learn what really lurks in the mind of a killer.