In the popular imagination MI5, or the Security Service, is known chiefly as the branch of the British state responsible for chasing down those who endanger national security-from Nazi fifth columnists to Soviet spies and today's domestic extremists. Yet, working from official documents released to the National Archives,distinguished historian Caute discovers that suspicion also fell on those who merely exercised their civil liberties, posing no threat to national security. In reality, this 'other history' of the Security Service, was dictated not only by the consistent anti-Communist and Imperial aims of the British state but also by the political prejudices of MI5's personnel. The guiding notions were 'Defence of the Realm' and 'subversion.'

Caute here exposes the massive state operation to track the activities and affiliations of a range of journalists, academics, scientists, filmmakers, writers actors and musicians, who the Security Service classified as a threat to national security. Guilt by association was paramount. Letters were opened, phones were intercepted, private homes were bugged and citizens were placed under physical surveillance by Special Branch agents.

Among the targets of surveillance are found such prominent figures as Arthur Ransome, Paul Robeson, J.B. Priestley, Kingsley Amis, George Orwell, Doris Lessing, Christopher Isherwood, Stephen Spender, Dorothy Hodgkin, Jacob Bronowski, John Berger, Benjamin Britten, Christopher Hill, Eric Hobsbawm, Kingsley Martin, Michael Redgrave, Joan Littlewood, Joseph Losey, Michael Foot and Harriet Harman. More than 200 victims are listed here but further MI5 files will be released to the National Archives.