A retired senior police officer describes the practical work of criminal investigation and counter-espionage during the First World War and its aftermath, blending procedural detail, memoir, and case studies. He explains organisational methods at Scotland Yard, everyday detective tradecraft, and the role of the Special Branch, then recounts prominent spy trials, wartime intrigues, women's involvement in espionage, and political unrest at home and abroad. The narrative moves from specific investigations and executions to reflections on morale, deception, and the challenges of postwar security and diplomacy, closing with observations on the return to normalcy after wartime anxieties.