MI5 is arguably the most secret and misunderstood of all the British government departments. Its enigmatic title - much more than its proper name, the Security Service - stands in the public mind for the dark world of the secret services in general. In reality it has a very specific brief: counterintelligence. Its object is to combat espionage and subversion directed against the UK.

Nigel West's book traces the history of MI5 clearly and accurately from its modest beginnings in 1909 until 1945, with the main part of the book focusing upon the important role which MI5 played in the Second World War. This includes the story of the sixteen enemy agents who were rounded up in Britain who were either hanged or shot; the manipulation of the Axis espionage networks by the use of 'turned' Abwehr agents (the famous Double Cross System), and the all-important check on its success provided by the intercepted German signals so brilliantly decoded at Bletchley; and the various deceptions practiced on the German High Command.

The book, which is laced with true anecdotes as bizarre and compulsively readable as any novel, is the fruit of years of painstaking research in the course of which Nigel West has traced and interviewed more than a hundred people who figure prominently in the story: German and Soviet agents, counterintelligence officers and, most remarkably, more than a dozen of the double agents.

In this new and revised edition, Nigel West details the organizational charts which show the structure of the wartime security apparatus, in what is regarded as the most accurate and informative account ever written of MI5 before and during the Second World War.