It's 1972, and local MP Ivan Cooper has organised a peaceful civil rights march in Derry, Northern Ireland. But chaos descends when a breakaway group of youngsters taunt British soldiers, and the body count rises.
Political uproar is guaranteed over this depiction of one of the most tragic chapters in the recent history of Northern Ireland. That's because, with a charged subject like this, individual viewers will see only what they want to see and ignore the rest. The facts, however, are incontestable: on January 30, 1972, 13 unarmed civilians were killed by British soldiers while taking part in a civil rights march in Derry. It's how the blame is laid on either side that provokes controversy.
Writer-director Greengrass - a World In Action veteran who also made TV docudrama The Murder Of Stephen Lawrence - aims for a gut reaction, drawing the audience into the fray with documentary-style, hand-held cameras. But it's Nesbitt's career-best performance that sets the tone for the audience to follow. As he shifts from cocky politician to ashen-faced witness, he helps the audience to understand that this is a human tragedy that goes beyond the sectarian divide.
Greengrass has deliberately chosen to provoke an emotional rather than intellectual response with his film - but sometimes provocation is much preferred to the British media's maintenance of the status quo.