At the height of World War I in April 1917, two young British soldiers, Lance Corporal Will Schofield (George MacKay) and Lance Corporal Tom Blake (Dean-Charles Chapman), receive a critical mission directly from General Erinmore (Colin Firth). They must deliver a message to the 2nd Battalion of the Devonshire Regiment, effectively calling off their planned attack on the German forces. The Germans have tricked the Allies into thinking they are retreating, setting up formidable defenses along the Hindenburg Line, which is laden with artillery and traps designed to ambush the battalion of 1,600 men, including Blake's brother Lieutenant Joseph Blake (Richard Madden). With no functioning phone lines, this urgent task falls squarely on the shoulders of Schofield and Blake.
Despite Schofield’s desire to wait until dark to set off, Blake's urgency to save his brother pushes them to depart immediately, setting off on a trek of 9 miles that should take at most 6 to 8 hours. They begin their perilous journey from a trench commanded by Lieutenant Leslie ([Andrew Scott]), who, skeptical of Erinmore’s assessment, warns them that it may be a trap but provides them the safest route across No Man's Land.
As they traverse the dangerous terrain, Schofield suffers an injury to his left hand while navigating through barbed wire. Their exploration of abandoned German trenches reveals a horrifying trap; an explosion triggered by a rat nearly takes Schofield’s life, but Blake acts swiftly to save him. Their adventure leads them to a deserted farmhouse where they rescue a downed German pilot trapped in the wreckage. Amidst the chaos, however, tragedy strikes when the pilot stabs Blake, leading Schofield to shoot the assailant and rush to comfort his dying friend, promising to complete the mission and inform Blake’s mother of his bravery.
Eventually, Schofield is picked up by a British unit led by Captain Smith ([Mark Strong]), who warns him of the callous nature of some men under fire, fearful that Colonel Mackenzie ([Benedict Cumberbatch]) may refuse to halt the impending attack. A shattered canal bridge near Ecoust-Saint-Mein complicates matters further, forcing Schofield to continue alone. He encounters hostile fire from a sniper, resulting in a tense standoff that leaves him unconscious after being hit in the helmet.
Upon waking at night, Schofield navigates through eerily illuminated ruins, before meeting a French woman with a baby who offers to help him. Despite her urgings, he realizes that time is not on his side and ventures onward. In a desperate act of survival, he kills a German soldier and narrowly escapes by jumping into a river, carried away by the current until he finds himself free from danger.
With dawn approaching, Schofield finally reaches the 2nd Battalion, only to discover that the attack is already underway. He desperately seeks out Colonel Mackenzie, pushing through a throng of soldiers to deliver the message. As chaos erupts around him and the infantry rushes into battle, Schofield manages to reach Mackenzie, who begrudgingly cancels the assault after reading the urgent letter.
In the aftermath, Schofield finds Joseph and, struggling with the weight of his revelations, relays the heartbreaking news of Tom’s death, returning his brother’s rings and dog tags. Even amidst their grief, Joseph expresses gratitude for Schofield’s efforts. Exhausted, Schofield takes a moment to rest under a tree, reflecting on his family as he contemplates the profound message written on a photograph’s back: > “Come back to us.”
