Britain's Secret Intelligence Service (also commonly known as MI6) was born a century ago amid fears of the rising power of other countries, especially Germany. The next forty years saw MI6 taking an increasingly important role in shaping the history of Europe and the world. MI6's early days were haphazard but it was quickly forged into an effective organization in the crucible of World War I. During these war years, MI6 also formed ties with the United States -- harbingers of a relationship that would become vital to both countries' security as the century progressed. These early years also saw the development of techniques that would become plot devices in a thousand books and films: forgery, invisible ink, disguises, concealing mechanisms, and much more. The interwar years were nominally peaceful, but Britain perceived numerous threats, all of which MI6 was expected to keep tabs on. The outbreak of World War II once again caught MI6 off balance, and high-profile blunders (and the memoirs of MI6 operatives such as Graham Greene) created an impression of ineffectiveness. At the same time, however, the service was pioneering cryptography at Bletchley Park (where the Enigma code would be broken) and devising the very methods and equipment that would inspire Ian Fleming's novels. In a way, the aftermath of World War II was as dramatic as the war itself had been, because 1945-49 saw not only the end of the British Empire but also the emergence of a new sort of conflict: the Cold War. We witness MI6 wrestling with these epic developments as it tightens its bonds with the newly christened CIA, changes that would dictate the shape of the service -- and the world -- for decades to come.