Inspired by the story of the forty-seven rōnin. If you don't know it, then don't look that up. I can't give a description without spoiling it. Consider yourself lucky to have a fresh mind.
This is a patient yet tense movie. I have next to no notes, because there's really nothing to write or even think about. Recommended, though not "highly recommended", as there are a lot of movies which execute this general manly-man theme extremely well. Though it's not among its number, anyone who likes the samurai genre should give this a watch.
As a commentary on Edward Snowden or WikiLeaks or some such, a list, that somehow exists, is out in the clear and being hunted by various spies.
It would have made a huge difference had this not been portrayed as a flashback/debriefing. Because of this complete failure in storytelling, there was no sense of risk at all; an important part of a spy movie like this.