During World War I, the soldiers of the British 5th Battalion's Y Company under Capt. Bramwell Jennings Laurence Fox push into a grim trench assault, only to find the battlefield quickly devolving into a tests of nerve and fate. The mission centers on the hesitant but determined Pvt. Charlie Shakespeare Jamie Bell, who is nudged forward by Sgt. David Tate Hugo Speer as the company threads through fog, shellfire, and the claustrophobic maze of dugouts. The group fights its way forward, but what begins as a tactical push turns into a haunting ordeal as machines guns and artillery swallow comrades, leaving the survivors scattered, exhausted, and suddenly deeply isolated in the choking mud.
Carrying Pvt. Colin Chevasse Rúaidhrí Conroy on a stretcher, the unit stumbles into a maze of trenches that seems to stretch into infinity. They encounter three terrified Germans who react with a strange, almost parasitic fear as if something otherworldly lurks deeper within the earth. In a tense exchange, Thomas Quinn Andy Serkis shoots one of the Germans, another flees, and the third, Friedrich Torben Liebrecht, surrenders. The incident convinces the men they have breached enemy lines, and they resolve to hold the trench, thinking they have emerged into a fragile moment of control. Yet the trove of unsettling discoveries follows: bodies decayed and strangled by barbed wire, abandoned kits frozen in time, torn watches and broken compasses—an eerie symmetry with the soldiers’ own unraveling morale. When detonations close passages, a demon-like growl rumbles through the mud and blood stains the earth, hinting at a danger far beyond conventional combat.
The fighting intensifies as Pvt. Jack Hawkstone [Hans Matheson] is ambushed by a second German hidden in the ooze. He and Pvt. Barry Starinski [Kris Marshall] falter, and Thomas even executes the captured foe with a pistol, a brutal act that unnerves the already fragile unit and draws the ire of the others. When radio contact with command reveals that White Company may already have been wiped out, the remaining soldiers must shoulder the burden of leadership in a place where rules crumble and fear becomes a weapon of its own.
As night falls, the mood turns heavier. Cpl. Doc Fairweather [Matthew Rhys] tends the wounded, but the sense of menace grows; Barry keeps his distance, then stumbles upon three German corpses wrapped in barbed wire, positioned upright as if mockingly saluting the living. One of the corpses suddenly animates, attacking. Charlie and Tate arrive to find Barry’s corpse suspended by wire, a brutal reminder that something unseen is manipulating the trench from within. When they force Friedrich to speak, he reveals that the others turned on each other and that there is indeed something evil inhabiting the trenches, a force that seems to feed on fear and fracture.
Morning brings a brittle calm that shatters quickly. Jennings’s leadership fractures under stress, and the men clear the dugouts with grenades while the noises of artillery and advancing infantry echo as if the earth itself is moving. In a moment of tragic misperception, Jack is mistakenly shot in the chaos. Despair festers into distrust, and Willie McNess fantasizes about desertion, his nerves unraveling with every echoing step through the trenches. That night, a red mist drifts through the line, voices whisper, and Willie is drenched in blood, chased into no-man’s-land, and finally killed by Bradford after a desperate chase. Doc attempts a rescue but Willie’s condition worsens, and he drags himself toward safety only to be pulled under by something tunneling below.
Anthony Bradford [Hugh O’Conor] enters the narrative more directly as madness tightens its grip: convinced that both the trench and death itself are possessed, he pleads with Charlie to shoot him so he won’t kill again. When Charlie refuses, Anthony leaves, ultimately dragging Doc into a trap of barbed wire and forcing a grim choice on the chaplain. The tension between duty and mercy seems to fracture with every breath, as the men either act or wither under the pressure.
The next day casts a brutal crescendo. Thomas crucifies Friedrich alive on a wooden beam in no-man’s land and is himself stabbed in a fevered, frenzied struggle with Tate, who becomes entangled in barbed wire. Thomas ultimately kills Tate and taunts Charlie when the latter refuses to shoot him, only to be impaled by living strands of wire that rise from the mud. Charlie must decide whether to end his comrade or risk becoming a casualty of the trench’s ravenous, inexplicable power. He ultimately frees Friedrich and arms him with a rifle to defend himself, then rushes toward help.
In a heartbreaking turn, Charlie discovers Colin, pale and covered in flies, whose legs have already been gnawed by rats. Charlie’s tears mark a devastating realization: the boy’s legs are gone, and he must euthanize him. The tension escalates as Anthony ties Doc up with barbed wire and implores Charlie to kill the chaplain as well. Charlie’s resolve wavers, and Anthony shoots Doc, leaving Charlie to confront the moral weight of mercy versus survival. In a final, grim reconnaissance, Charlie chooses to end Anthony’s rampage with his own hands, a moment that seals the fate of the unit in a trench where pain and doubt have become the only constants.
The trench’s earth begins to cave, passages become blocked by the relentless embrace of barbed wire, and Charlie is dragged into a subterranean labyrinth. He wakes in a cave filled with corpses and sees living versions of his men, including himself, sitting around a macabre campfire of memory. Panicked, he escapes back to the trench—and there, Friedrich, now healthy and calm, points his rifle toward him with quiet menace. Friedrich’s offer to let Charlie go, and his enigmatic remark about freedom, suggests a return to the surface is possible, but at a terrible price. The ladder back to no-man’s-land appears, inviting Charlie to climb toward an uncertain fate as the fog swallows the trench and the world above.
In the aftermath, a fresh team of British soldiers arrives at the slaughtered trench. They find Friedrich sitting alone, a chilling calm in his expression as he lifts his hands to surrender, the same gesture of submission that began the nightmare turning into a possible repeat. The film ends on this ominous note: the cycle, the trench, the evil, and the choices made in the murk of war ready to recur, as if history itself is condemned to relive the same horrors over and over again.
