This memoir enters Room 408 of historic Scotland Yard, where spies, international swindlers, and revolutionaries were questioned during the First World War. Sir Basil Thomson replaces the mythology of fictional detectives with the patient, difficult reality of police work. From the interrogation of Mata Hari and the breaking of secret codes to condemned spies facing death, Thomson looks beyond individual crimes to fear, mass hysteria, and the psychology of people living near the edge of the law. His account combines investigative rigor, dry humor, and moments of unexpected human sympathy.