DIRECTOR Gavin Hood has a way of making seemingly mundane actions look momentous. Eye in the Sky (2015), about a drone attack on suspected terrorists, relied on Helen Mirren watching a bank of screens as she controlled a military operation thousands of miles away. In Official Secrets, the posting of a letter is electrifying.

The woman armed with the envelope in this case was Katharine Gun. Played here by Keira Knightley, Gun was the GCHQ translator who blew the whistle on a US attempt to pressure members of the UN Security Council into backing the invasion of Iraq in 2003. This is her story, thrillingly told.

Gun is seen watching Tony Blair being interviewed on television by Sir David Frost. The then PM states he has no doubt that Saddam has weapons of mass destruction. “Just because you are Prime Minister doesn’t mean you get to make up your own facts,” Gun exclaims.

Gun does a lot of shouting at the television, as did many people in those anxious days of Bush-Blair press conferences, dodgy dossiers, and stop the war marches. Living a quiet life with her cafe manager husband in Cheltenham, Gun is presented as an ordinary woman who just happens to also be a spy, albeit her work is done on a computer rather than out in the field.

When an email from the US National Security Agency arrives asking for dirt on UN Security Council members, Gun decides she has to do something. From there the story switches to The Observer, the paper that broke the story, and then on to the courts. The screenplay has a lot to pack in, from a history of the Official Secrets Act and whistleblowing to the workings of the UN, and the exposition does hang heavy at times.

The Observer scenes provide some comic relief, with Rhys Ifans hamming it up as a maverick reporter and Matt Smith playing the gentleman journalist who dares to run with the story while others fear the memo is a fake. All are based on real people, some of whom might want to see something else at the cinema this weekend. Journalism, like many another institution of the time, did not exactly cover itself in glory, a fact reflected here.

Knightley is terrific as the mouse that roared. Though the film does raise questions about Gun’s actions (why should she get to decide which secrets to keep?), she is celebrated as a heroine. While this runs the risk of turning her into a sainted figure, Knightley makes her only too human by showing her doubts, fears, and naivete, as well as her undoubted courage.

Spread the word – this is an ace political thriller.