That’s some catch, that Catch-22.

Unless you’re British, of course. There the biggest catch of them all might be “The Official Secrets Act,” a sweeping anti-leaks law that makes the mere act of defending yourself from charges that you let state secrets out of your hands is pretty much illegal.’

Non-disclosure forms mean you can’t even discuss what you do for the government with your lawyer, preparing your defense.

Not that this stopped Katherine Gun. As played by Keira Knightley in “Official Secrets,” she’s a woman of conscience and considered impulse, who’d shout at the TV coverage of the rushed, relentless and patently dishonest rush to war with Iraq in 2003.

“BLOODY LIAR!”

Bush or Blair or Colin Powell, it didn’t matter. When you’re in “sig-int” (signals intelligence) and you can read emails, dispatches and communiques from U.S. National Security Agency officials detailing how facts will be bent, foreign countries will be blackmailed and other chicanery used to justify George W. Bush’s war, a civil servant like Kat Gun is on the horns of a dilemma. Allow this to go on, or expose this “illegal war” (as detailed in British law) by any means necessary.

The latest political thriller from Gavin Hood, the South African director who gave us “Tsotsi,” “Rendition” and “Eye in the Sky” (an a “Wolverine” movie) is an arcane, somewhat static docu-drama about the real-life struggles of Kat Gun. She did not come to her “treason” easily. And it’s about the wary and tentative way the Observer newspaper and reporter Martin Bright (former Doctor Who Matt Smith) approached the leak, and “official” acts to intimidate and punish her and — a separate issue — actually prosecute Gun under the broad “Official Secrets Act.”

The film isn’t quite as dry as a history lecture, enlivened by some empassioned performances by an A-list of British character actors, including Smith, Ralph Fiennes (an activist lawyer), Tamsin Greig (an editor), Jeremy Northam (head of the office charged to prosecute), and Matthew Goode and Rhys Ifans as Jekyll and Hyde reporters coming at the story from different perspectives.

Politically, you can see why they signed on. It’s an important story that roiled Britain for a decade and forever stained the reputation of Tony Blair. For Americans, seeing all the intercut news footage of speeches, U.N. testimony and the bums’ “rush to war” this is about, it’s a reminder to banish the phrase, “W. wasn’t so bad, was he?” from conversation.

Knightley plays the real-life Gun, a translator who wrote reports summarizing the intelligence she was focused on, as increasingly enraged about the news coverage she could not tear herself away from at home with her Muslim immigrant husband (Adam Bakri).

She knows there are no ties between Iraqi dictator Saddam Hussein and Al Qaida, and at some point she’s going to stop venting “It doesn’t make any bloody sense!” to her mate and get up the gumption to get some proof out to the public.

“At least my conscience will be clear” isn’t what the intermediary (MyAnna Buring) recieving this printout wants to hear. There’s prison awaiting anybody who touches it.

And even after reporter Bright talks his “We officially ‘support the war'” editor (Conleth Hill) into publishing the bombshell, it’s not like history will be changed.

“Give me 400 words the average reader will undrstand!”

Online American conservative war boosters like Matt Drudge devoted themselves to debunking the leak.

Then come the “official” interrogations.

What Hood’s movie does best is capture the contrast between Gun and the likes of Edward Snowden, a woman of conscience who eventually faced the music over her actions. You’re more credible if you get a lawyer and take your fight into court.

Knightley gives us Gun second-guessing herself, quaking in fear at what is facing her and her perfecfly-deportable husband, and then flashes of righteous fury about why she did it and the law-breaking she was exposing.

She’s always good, and sometimes she’s just dazzling.

Fiennes has a nicely-pitched wariness in his “Liberty” (a British “innocence project”) lawyer, part of an entitled class of high-end barristers who all (even his class-friend, official foe, Northam) weekend in their cottages on the coast.

Ifans is the shouting sound and fury of an activist reporter, Ed Vulliamy, Goode nicely contrasted as the connected, cautious one sent out to get “confirmation” of this or that when Ifans’ Vulliamy can’t be trusted with that task, word “from a good friend of a strusted friend…”

Smith’s Bright is the least interesting in the lot, as earnestly but drably played by the actor. But even he has his  moments.

There’s plenty of arcane intel slang for spy thriller geeks, the inevitable oblique conversations with “We didn’t exactly have a conversation” insiders. Anybody used to the masterworks of John LeCarre will recognize the tone, the tiny drips of exposition adding up to a puddle.

The arcana extends to questions of British law and press freedoms, which are different from what we see in the U.S.

The film touches on class, politics, journalistic ethics (and blunders) and the natural skepticism any reporter must bring to a story that fits your own politics or narrative. It’s pace is rather too much like the British TV “State of Play” to be a lively big-screen experience. But as somebody who loves intel arcana and journalistic debates, I found much to enjoy here.

“Official Secrets,” despite its blasé title, despite the fact that this “true” story isn’t on a LeCarre level, in spite of its paucity of dramatic outbursts, is still a most engrossing history reminder.

Whatever forces in America, from the Bush Administration to its Dixie Chicks-banning media minions, were doing to march us off to war, in Britain the vast majority were against the coming slaughter. And there were civil servants and journalists willing to speak out, no matter what “Catch-22” might await them.

MPAA Rating: R for language

Cast: Keira Knightley, Ralph Fiennes, Matt Smith, Rhys Ifans, Adam Bakri, Matthew Goode and Jeremy Northam

Credits: Directed by Gavin Hood, script by Gregory and Sarah Bernstein, and Gavin Hood based on a book by Marcia and Thomas Mitchell.  An IFC/eOne release.

Running time: 1:52