On 6 April 1917, aerial reconnaissance has observed that the German army, which has pulled back from a sector of the Western Front in northern France, is not in retreat but has made a strategic withdrawal to the new Hindenburg Line, where they are waiting to overwhelm the British with artillery. In the British trenches, with field telephone lines cut, two young British soldiers, Lance Corporals William Schofield, a veteran of the Somme, and Tom Blake, are ordered by General Erinmore to carry a message to Colonel Mackenzie of the Second Battalion of the Devonshire Regiment, calling off a scheduled attack that would jeopardise the lives of 1,600 men, including Blake's brother Lieutenant Joseph Blake.

Schofield and Blake cross no man's land to reach the abandoned German trenches. In an underground barracks, they discover a booby-trap tripwire, which is promptly triggered by a rat. The explosion almost kills Schofield, but Blake saves him, and the two escape. They arrive at an abandoned farmhouse, where a German plane crashes after a dogfight with Allied aircraft. Schofield and Blake drag the burned pilot from the plane. However, the pilot stabs Blake and is shot dead by Schofield. Schofield comforts Blake as he dies, promising to complete the mission and to write to Blake's mother. Schofield is then picked up by a passing British unit.

A destroyed canal bridge near Écoust-Saint-Mein prevents the British lorries from crossing. Schofield chooses to part with them at the bridge. He then uses what is left of the bridge to cross alone, and quickly comes under fire from a German sniper. He and the sniper shoot each other simultaneously; the sniper is killed, while Schofield is knocked unconscious. He regains consciousness at night, and finds the town in flames. He encounters another German soldier, and escapes by hiding in the basement of an abandoned building, where he stumbles into the hiding place of a French woman with an infant. She treats his wounds, and he comforts the infant by reciting "The Jumblies" by Edward Lear, giving the woman his canned food and milk from the farm. Despite her pleas, Schofield leaves soon after, realising that it is morning and time is running out. After strangling one German soldier and pushing past another who is inebriated, he escapes by jumping into a river. He is swept over a waterfall before reaching the riverbank. In the forest, he finds D Company of the 2nd Devons, which is in the last wave of the attack. As the company starts to move toward the front, Schofield tries to reach Colonel Mackenzie.

Realising that the trenches are too crowded for him to make it to Mackenzie in time, Schofield sprints across the open battlefield, just as the infantry begins its charge. He forces his way into meeting Mackenzie, who reads the message and reluctantly calls off the attack. Mackenzie says that, while the cancellation offers a temporary reprieve, command will likely change its orders in a week. Schofield is told that Joseph was in the first wave, and he searches for him among the wounded, finding him unscathed. Joseph is upset to hear of his brother's death, but thanks Schofield for his efforts. Schofield gives Joseph his brother's rings and dog tag, and asks to write to their mother about Blake's heroics, to which Joseph agrees. Exhausted, Schofield sits under a tree, looking at photographs of his wife and two daughters.