Juan Pujol, a young Spanish anti-fascist, ultimately became Agent GARBO -- the greatest double-agent of World War II. Initially recruited by German intelligence, GARBO came to London after a series of adventures to work for the British. Using a ring of invented sub-agents, he and his MI5 controllers eventually succeeded in pulling off one of the greatest deceptions in history. As a part of Operation FORTITUDE, they convinced the Germans that the D-DAY landings were only a diversionary attack, so protecting the Allied landings and hastening the end of war in Europe. The release of MI5 case files (kept secret for over fifty years), means that these facts can now be told in full. The report which forms the bulk of this publication reads like a classic spy adventure -- enciphered messages, secret inks, items concealed in cakes, exotic foreigners and fanatic nationalists. It was written by GARBO'S MI5 case officer, Tomas Harris, one of a group of Cambridge graduates that included the Soviet spies Anthony Blunt and Kim Philby. Mark Seaman's introduction and notes explain and illustrate the crucial historical importance of this MI5 file.