Paul Greengrass’s approximately real-time dramatization of what took place aboard Flight 93—which left Newark for San Francisco the morning of September 11, 2001, and crashed in western Pennsylvania 81 minutes after takeoff —is best understood as a memorial. Like most memorials, it is respectful, premised on competing obligations to the dead and the living, and eager to stress that the deaths were not in vain. As written and directed by Greengrass, the ex–BBC documentarian who already has one skillful re-creation of a historical atrocity under his belt ( Bloody Sunday, about 1972’s Derry massacre), United 93 is at once scrupulous and ghoulish, visceral and sober. Perhaps mindful of his target audience, Greengrass makes sure to dangle some red-state red meat. In the blurry rebellion that is the film’s raison d’être—a spoiler follows—the passengers appear to kill two of the terrorists. It’s the most problematic of the movie’s unverifiable events, and one might say its biggest concession to popular taste. Painful as this movie is, it’s even more excruciating to imagine how it might play in some of the country’s multiplexes.