Plot

At the height of the First World War in April 1917 in northern France, two young British soldiers, Lance Corporal Will Schofield (George MacKay) and Lance Corporal Tom Blake (Dean-Charles Chapman), are given a mission directly by General Erinmore (Colin Firth) to hand-deliver a message to the 2nd Battalion of the Devonshire Regiment, calling off their planned attack on the German forces. The Germans have feigned retreat to the Hindenburg Line (it is 3 miles deep, artillery, defenses and fortifications never seen before) and are prepared to ambush the battalion of 1,600 men, Blake's brother Lieutenant Joseph Blake (Richard Madden) among them. Erinmore has aerial photos that show the German fortifications which will destroy the advance of the Devonshire Regiment, who aims to attack at dawn the following morning. As all phone lines are cut there is no other way to get the message across.

Schofield wants to wait till dark, but Blake wants to save his brother and leaves immediately. They are 9 miles from their destination. It should take them 6-8 hours to get there at the most. Lieutenant Leslie (Andrew Scott) is in command of the trench from where the duo have to access the No man's land. Leslie doesn't believe Erinmore's analysis and says that the Germans are still manning their trenches. He calls it a trap but shows them the best way across the No Man's Land.

Schofield and Blake cross no man's land to reach the abandoned German trenches, but Schofield injures his left hand along the way (when it snags in the 2nd line of barbed wire). In an underground barracks (the duo finds massive under bunkers in the German trenches with cots for soldiers to sleep), 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 (Schofield fills his water flask with milk he finds in a bucket), 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 (led by Captain Smith (Mark Strong)) and dropped off near the bombed-out village of Ecoust-Saint-Mein. Smith tells Schofield to have witnesses when he gives the orders to Mackenzie. Smith knows that some men just want a fight and Mackenzie may not call off the attack even after being ordered.

A destroyed canal bridge near Ecoust-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 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 realizing that time is running out. Encountering German soldiers who were occupying Ecoust, 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.

Schofield reaches the 2nd Battalion in the morning only to find that the attack has already begun, and that Blake's brother is among the first wave to go over the top. As the company starts to move toward the front, Schofield tries to reach Colonel Mackenzie (Benedict Cumberbatch).

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 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. Exhausted, Schofield sits under a nearby tree and looks at photographs of his family; on the back of one of them is the message Come back to us.