Exacting and meaty narrative is fascinating tedium.

True story based on the book by Marcia and Thomas Mitchell presents an eye-popping dilemma: the conflict between loyalty to country and contractual obligations. In the lead up to the Iraq War, an analyst for the British government named Katherine Gun (Keira Knightley) leaked a highly confidential email, which made its way to the press. In the investigation that followed, Gun was discovered and prosecuted for a violation of the UK’s Official Secrets act.

The film, told primarily from Gun’s point of view, casts her as a patriot, whose goal was not for personal gain. She was not working for a foreign entity and received no financial renumeration for her actions. However, some in the government believed that she leaked the email in order to build her celebrity. And, if the facts are consistent with the story told in this movie, Gun’s motivations were genuine and sincere.

Matthew Goode and others make up a superb supporting cast.

A frightening, real-world thriller, director Gavin Hood, who last gave us 2015’s excellent “Eye in the Sky,” has a good feel for the material. The direction is tight and Hood’s assembled a veritable “who’s who” of British acting talent led by a completely devoted Knightley as Gun and anchored by a suitably buttoned up Ralph Fiennes, as the attorney who defends her. It’s a talky affair, but one that is really informative and revealing, if, also, tedious.

A few supporting players melt into their characters in this ensemble. Matthew Goode is positively unrecognizable as a journalist, who works with Matt Smith and others. Their editor is played by “Game of Thrones” regular Conleth Hill, and Rhys Ifans chews scenery as an unhinged, crusty investigative reporter.

Knightley is thoroughly devoted to her role here.

The depth of the cast is impressive and keeps things humming along. The movie pulls together elements of other films, like “The Post” and proper legal thrillers. The story, which played out in the media years ago, might have a little hair on it, but it’s still engrossing and important. Kudos to Knightley for continuing to take on complex roles. Those who dislike her and are sniffy about her abilities are just wrong.

In all the talk about various legal defenses and reasoning, one thing bothered me: isn’t there a legal way to expose possible wrong-doing by your government? In this country, we have whistleblower laws that provide a mechanism. However, the question would always be whether that system provides sufficient protection and ensures that the right thing will be done. National security as a justification for suppressing speech has its limits.