To chronicle the events of the morning of September 11, 2001, on film is to accept a set of inescapable constraints. It is to venture onto sacred ground bearing a daunting burden of responsibility—to the facts, to the sentiments of the immediate families, to the American mood, to the sensitivities of a still fraught and roiling national psyche. It’s hard to imagine many filmmakers capable of walking this line and not succumbing to the temptations of mythmaking and monumentalism, but anyone who saw 2001’s Bloody Sunday would know that English docudrama director Paul Greengrass belongs to the select group of individuals up to the task.

A stark, wrenching, and overwhelming viewing experience, shot in a cinema vérité style that becomes increasingly fragmented as events accelerate, United 93 is a film of two interwoven parts; its first half is primarily concerned with depicting what went wrong on the ground, offering a riveting and meticulous inside view of the appalled helplessness of those manning the Eastern Seaboard’s air-traffic-control system and the failure of the air defense chain-of-command. Devoid of sensationalism, the film’s second part details the hijacking of United Airlines Flight 93 by five Muslim terrorists—and the passengers’ subsequent struggle to retake the aircraft by force. Greengrass’s version of what transpired aboard the Boeing 757, extrapolated from the behavioral profiling of each person on the plane and the eyewitness reports given by the jet’s passengers using cell and air phones, is ultimately the product of well-considered, rigorously conscientious speculation. By contrast, the events down below are reenacted, with a number of real-life participants playing themselves, most notably the head of the National Air Traffic Control Center, Ben Sliney, the man at the eye of the 9/11 hurricane.

In a very real sense, Greengrass hasn’t simply accepted the intrinsic constraints of this undertaking, he’s embraced them, boxing himself in even more by entrenching formal guidelines that serve as aesthetic counterparts to the moral obligations built into the subject matter. Hence the 91-minute flight is shot in real time and, a few brief preliminary scenes aside, the film’s scope is narrowed down to a handful of settings: five windowless control rooms and the inside of the passenger jet. It’s a film composed entirely of interiors, of totally controlled environments in which control is irrevocably usurped. Aside from a bird’s eye view of Manhattan at night in the film’s opening moments, Greengrass permits himself to shoot only from camera positions that can be justified by the conceivable presence of a human observer—we never see the exterior of the plane once it’s airborne, and the devastation of the Twin Towers is seen only from the remove of the Newark Airport control tower and the television screens in the various control rooms.

Allowing only the briefest (but nevertheless gasp-inducing) glimpses of catastrophe—the distant form of an airliner flying low across the horizon towards Manhattan; the ground rushing up to meet a plane locked into an irreversible nosedive—United 93 can be said to present the events of 9/11 strictly as a series of abstractions unfolding on the radar screens and monitors of air-traffic control centers and in the operational chatter of the men and women staffing them. As such, its moral stance is that of a somber, uncompromising anti-spectacle.

Greengrass’s film is many other things besides: a cathartic act of bearing witness, an experiment in therapeutic reenactment, an anti-procedural, a meditation on the agonizing limits of communication—and a memorial.

When all is said and done, Greengrass got it right.