Compared to many of MI5's other double agents, HARLEQUIN’s career was very short-lived, lasting only for a few months in 1943. However, during that time he provided insights into the various parties involved in the Appeasement process in 1938; the Czech crisis of 1939; the enterprises of a Franco-American businessman who hosted the Duke and Duchess of Windsor’s marriage in France; the espionage activities of an aristocratic German family; Admiral Canaris, the head of the Abwehr – many of the Abwehr’s personalities with whom he had come into contact or had known about and the agents he employed – as well as relations between the disparate organisations of the German intelligence services – the Abwehr, Gestapo, and Sicherheitsdienst (SD), the intelligence arm of the SS. Furthermore, he revealed the German Armistice Commission’s involvement in espionage and their links to the Abwehr. MI5 shared this intelligence with the FBI and the Office of Strategic Services (OSS) before HARLEQUIN requested that he be returned to American custody where he remained for the rest of the war. His effectiveness as a double agent will be examined using newly-released official files as a primary source.