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 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 jeopardize the lives of 1,600 men, including Blake’s brother Lieutenant Joseph.
Schofield and Blake cross no man’s land to reach the abandoned German trenches. In an underground barracks, they discover a tripwire set by the Germans, 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 sniper. Exchanging shots, Schofield wounds the sniper and advances, 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 discovers a French woman hiding with an infant. She treats his wounds, and he gives her his canned food and milk from the farm. Despite her pleas, Schofield leaves, after hearing the chimes of a nearby clock and realizing that 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 D Company of the 2nd Devon’s, which is in the last wave of the attack. As the company starts to move toward the front, Schofield tries to reach Colonel Mackenzie.
Realizing that the trenches are too crowded for him to make it to Mackenzie in time, Schofield goes “over the top” and sprints on the open battlefield parallel to the British trench line, just as the infantry begins its charge. He forces his way in to meet Mackenzie, who reads the message and reluctantly calls off the attack. Schofield then finds Joseph, who was among the first wave and is bloodied but is unharmed. Schofield tells Joseph of his mission and that his brother Tom has died, passing on Tom’s rings and dog tag. 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 nearby tree, looking at photographs of his wife and children.