'Official Secrets'

As based-on-a-true-story movies go, “Official Secrets” feels more real than most, a credit to the British penchant against hyperbole and the understated performances of its cast — particularly its star, Keira Knightley.

Knightley plays Katherine Gun, who in early 2003 — on the eve of the invasion of Iraq — was a signals analyst for GCHQ, the British government’s intelligence-gathering agency. In short, as a character notes later, she’s a spy, charged with gathering, processing and analyzing data for Her Majesty’s government.

One morning, everyone in Katherine’s office receives a memo from GCHQ’s American counterpart, the National Security Administration. It’s a general call for intel that can be used toward — or, more accurately, against — countries sitting on the United Nations Security Council. The intent is clear: The Bush administration is looking for dirt that can be used to blackmail countries into approving a UN resolution supporting an invasion of Iraq.

Katherine has been discussing the run-up to war with her husband, Yasar (Adam Bakri), a Kurdish Iraqi who emigrated to the UK from Turkey, and how Tony Blair and his cabinet were lying their way into a war. When the memo arrives, Katherine feels compelled to do something, so she makes a copy, which she mails to an anti-war activist friend (MyAnna Buring).

The memo makes its way, eventually, to Martin Bright (played by Matt Smith), a reporter at The Observer, which has already declared an editorial position in favor of going to war. He and the paper’s DC correspondent (Rhys Ifans) try to nail down the veracity of the memo, and when they do, the paper publishes their story. But the American media thinks it’s a fake — for reasons both of politics and, as one editor (“Game of Thrones’” Conleth Hill) bellows, someone being “colossally stupid” — and the march to war continues.

Within GCHQ, an investigation into the leak begins, and Katherine, sickened that her coworkers may suffer under suspicion, confesses. Thus begins her ordeal for violating the Official Secrets Act, as the government holds charges over her head, spies on her, and threatens Yasar’s immigration status. When she goes to a civil-rights law firm, the lead barrister, Ben Emmerson (Ralph Fiennes), suggests a novel defense — but Katherine is warned that saying anything, even to her lawyer, about the memo would also violate the Official Secrets Act.

Director Gavin Hood has explored similar issues before, in “Rendition” (2007) and “Eye in the Sky” (2015), and here — as co-screenwriter with married scribes Gregory and Sara Bernstein — he sets an unfussy, just-the-facts tone, a police procedural of the security state. (Spoiler: The American government comes off looking quite badly, not that Blair is let off the hook.) The story has an offbeat structure, following the memo’s journey before settling on Katherine’s legal problems.

The strong ensemble cast also includes Matthew Goode as Bright’s fellow journalist, Indira Varma and John Heffernan as Emmerson’s colleagues, and such old-school British actors as Kenneth Cranham and Clive Francis in small roles.

But Knightley holds our attention throughout “Official Secrets,” capturing Katherine’s optimism that she can stop the war, her anger that she was naive for thinking she could, and her resolution to reveal the truth no matter the cost. It’s a sharp reminder of what lies a government will tell to get what it wants, and how some individuals find the strength to speak the truth.

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‘Official Secrets’

★★★1/2

Opened August 30 in select cities; opens Friday, September 13, in more theaters nationwide. Rated R for language. Running time: 112 minutes.