A look at the history of MI5 based on the book Defence of the Realm by Professor Christopher Andrew, who was given unprecedented access to MI5's top secret files. Featuring exclusive interviews with a former MI5 Director General and an ex-head of the Metropolitan Police's anti-terrorism branch, the film also reveals the truth about the alleged plot against Harold Wilson, and addresses allegations that the killing of three IRA agents in Gibraltar was proof of MI5 having a shoot-to-kill policy.