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 the next morning 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 tripwire, that the Germans set to kill anyone inside it, 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 is shot down in a dogfight with Allied aircraft. Schofield and Blake save the burned pilot, but 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. Taking Blake's rings and dog tag, as well as Erinmore's letter, he is picked up by a passing British unit.
A destroyed canal bridge near Écoust-Saint-Mein prevents the British lorries from crossing, and Schofield chooses to part with them. He uses what is left of the bridge to cross alone, and comes under fire from a German sniper. Exchanging shots, Schofield wounds the sniper and advances into his nest, whereupon he and the sniper shoot each other simultaneously; the sniper is killed, while Schofield is knocked out. He regains consciousness at night, and finds the town in flames. He finds a French woman hiding with an infant. She treats his wounds, and he gives her and the infant his canned food and milk from the farm. Despite the woman's pleas, Schofield leaves, after hearing the chimes of a nearby clock that it is morning and time is running out. Pursued by German soldiers after strangling one to death, he escapes by jumping into a river. He is swept over a waterfall before reaching the riverbank. In the forest, he finds the 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. Schofield then finds Joseph, who was among the first wave but is unscathed. Schofield tells Joseph of his mission and that his brother Tom is dead, passing on Tom's rings and dog tag to Joseph. Joseph is deeply upset about his brother but thanks Schofield for his efforts. Schofield asks to write to their mother about Tom's heroics, to which Joseph agrees. Exhausted, Schofield sits under a tree, looking at photographs of his wife and children.