Gary Hook, a new recruit to the British Army, takes leave of his much younger brother Darren. Hook's platoon is sent to Belfast in 1971 in the early years of the Troubles. Under the leadership of the inexperienced Second Lieutenant Armitage, his platoon is deployed to a volatile area where Catholic Nationalists and Protestant Loyalists live side by side. The unit provides support for the Royal Ulster Constabulary as it inspects homes for firearms, shocking Hook with their rough treatment of women and children. A crowd gathers to protest and provoke the British troops who, though heavily armed, can only respond by trying to hold the crowd back.
One soldier is hit by a rock and drops his rifle to the ground. In the confusion, a young boy seizes it and runs off through the mob. Hook and another soldier, Thompson, pursue him. As the crowd's protest violently escalates, the soldiers and police pull out, leaving the two soldiers behind. Hook and Thompson are severely beaten by a mob, until a sympathetic woman manages to calm things down. However, Thompson is suddenly shot dead at point blank range by the Provisional Irish Republican Army (PIRA) gunman Paul Haggerty. With the crowd physically attacking him, Hook flees through streets and back alleys, eludes his pursuers, and hides in an outhouse until dark.
A Protestant youngster brings Hook to a local pub that serves as a front for Loyalists. There, Hook glimpses a Loyalist group in a back room, constructing a bomb under the guidance of a member of the Military Reaction Force (MRF), the British Army's covert counter-insurgency unit. Hook steps outside the pub just before an enormous explosion destroys the building, killing or injuring many of those inside, including the young boy who brought him there. Hook flees once more into the dark streets. Unaware that the Loyalist bombers have blown themselves up accidentally, the PIRA and Official Irish Republican Army (OIRA) factions accuse each other of being responsible for the bombing.
Two Catholics, Eamon and his daughter Brigid, discover Hook as he lies in a street unconscious and injured by shrapnel. They take him to their home in the Divis Flats area before discovering he is a British soldier. Eamon, a former army medic, stitches Hook's wounds.
Despite the PIRA's having recently taken control of the area from the OIRA, Eamon contacts senior OIRA official Boyle for help, expecting a more humane solution than the PIRA faction would allow. Boyle, less radical and violent than the younger PIRA members, has a working relationship with the MRF and tells MRF Captain Browning, leader of the local MRF section, of Hook's whereabouts and asks in return that Browning kill James Quinn, a key leader of the PIRA faction.
Quinn and his PIRA squad have been tailing Boyle since the pub explosion and saw him visit Eamon's flat without knowing why he was there. Sensing danger, Hook flees the flat, taking an assault knife he finds in a bag. He eludes the PIRA men but, unable to evade Haggerty, Hook stabs and kills him.
Quinn's group captures Hook and takes him to a hideout. Quinn orders Sean, a young teenager, to murder Hook. When Sean hesitates, Quinn prepares to execute Hook, only to leave when Browning's group arrives. Sergeant Lewis of Browning's group shoots Sean, to Hook's horror. Lewis then attempts to strangle Hook to prevent him from informing others about the bomb.
As Lieutenant Armitage and his men enter in support of Browning, Armitage sees Lewis' attempt to kill Hook. Sean raises himself and shoots Lewis dead before being shot again, this time by Armitage. Browning finds Quinn, and rather than arresting him, tells him Boyle wants him dead, then lets him go. As Quinn leaves, Browning tells him he will be in touch soon, and he expects him to be helpful.
Hook is returned to his barracks. Later, despite a formal complaint by Armitage, the commanding officer dismisses the incident involving Hook, Lewis, and Sean as a confused situation that merits no further inquiry. Hook returns to England and reunites with Darren.
'71 received positive reviews. Rotten Tomatoes, a review aggregator, reports that 96% of 125 surveyed critics gave the film a positive review; the average rating was 7.7/10. The site's consensus reads: "Powerfully directed and acted, '71 stays true to its fact-based origins while remaining as gripping as any solidly crafted action thriller." On Metacritic it is rated 83/100 based on 33 reviews, indicating "universal acclaim".
Writing for the Los Angeles Times, Kenneth Turan called '71 "a tense thriller from Britain that so adroitly joins physical intensity, emotional authenticity and political acuity that you may find yourself forgetting to take a breath." Manohla Dargis of The New York Times singled out Jack O'Connell for praise, saying, "Mr. O'Connell runs away with '71, in which his character's every emotional, psychological and physical hurdle makes for kinetic cinema." The Hollywood Reporter critic Leslie Felperin noted of Yann Demange's direction, "A big part of [Demange's] achievement resides in the casting of such a veteran crew of character actors in the first place, but credit is due for coaxing such subtle performances." Jonathan Romney in Film Comment praised the originality of the film, "a rare hybrid between hard-nosed realism, on the cusp of a quasi-documentary style, and genre thriller-adventure", while criticising the opening and closing scenes as conventional.
'71 won Best Director at the 2014 British Independent Film Awards, after receiving nine nominations.
The National Board of Review named '71 one of the top 10 independent films of 2015.