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 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.

Schofield and Blake cross no man's land to reach the abandoned German trenches, but Schofield injures his left hand along the way. 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 from the wreck. Blake persuades Schofield to get water for the pilot. When Schofield's back is turned, the pilot stabs Blake. Schofield shoots the pilot dead and 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 struck in the helmet and knocked unconscious. He awakens at night and makes his way through the flare-lit ruins of the town. After evading a German soldier, 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 realising that time is running out. Encountering German soldiers, he strangles one to death and escapes pursuit by jumping into a river. The river carries him while the cherry blossoms fall. 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 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 looks for Blake's brother, and finds him, who was among the first wave and is bloodied but unharmed. Schofield informs Joseph of his mission and of Tom's death, passing on Tom's rings and dog tag. Joseph is deeply upset about his brother but thanks Schofield for his efforts. Schofield asks for permission to write to their mother about Tom's heroics, to which Joseph agrees.