1917

On 6 April 1917, aerial reconnaissance has observed that the Imperial German Army, which has pulled back from a sector of the Western Front in northern France, is not in retreat—as had been supposed—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 2nd 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 Joseph, a lieutenant.
Because travelling behind their own lines would take longer, Schofield and Blake cut across 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 crashed after being shot down in a dogfight with Allied aircraft. Schofield and Blake save the burned pilot from the wreck and attempt to help him. Despite this, the pilot fatally stabs Blake in the abdomen before Schofield shoots the pilot dead. Schofield comforts Blake as he dies from blood loss, 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 who have noticed the smoke from the burning plane. 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 confronts the sniper in his tower perch, whereupon he and the sniper shoot each other simultaneously; the sniper is killed, while Schofield is struck in the helmet and knocked unconscious.
Schofield awakens at night and makes his way through the flare-lit ruins of the town. Evading a small German platoon, he discovers a French woman hiding with a presumably orphaned 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 realising that time is running out. Encountering the platoon, Schofield strangles one and escapes pursuit 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 to form the last wave of the attack. As the company starts to move toward the front, Schofield rushes to reach Colonel Mackenzie. Realising 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 reluctantly reads the message and calls off the attack. Afterwards though, the jaded Mackenzie tells Schofield that command will likely change their tune tomorrow, further perpetuating a war of attrition.

Schofield finds Blake's brother, who was among the first wave but is unharmed. Schofield informs Joseph of his mission and of Tom's death, passing on Tom's rings and dog tag. He asks for permission to write to their mother about Tom's heroics, to which Joseph agrees. Exhausted, Schofield rests under a nearby tree and looks at photographs of his family.