During World War 1, and in the chaos of a charge across no man's land, a company of British soldiers is wiped-out by machine gun and artillery fire. Tate gets caught in barbed wire, and Shakespeare is unwilling to rescue him. The next morning, the survivors find themselves slowly advancing through a dense mist, which contrary to their fear is not a poison gas. As they emerge from it, the British come across a complex network of German trenches, where they find an apparently besieged handful of terrified German soldiers. Private Quinn shoots the first and attempts to shoot the second, but Private McNess stops him and the German stumbles down the trench. The third German is left, a young man named Friedrich. Convinced that they have broken through the enemy lines, the soldiers decide to secure the trenches. Doing so, they explore them and find ominous signs that something there has gone very wrong: they find rotting bodies everywhere, often wrapped in barbed wire. When detonating charges to close off some passages, they hear a demon like growl and as they walk away fail to notice vast amounts of blood pouring from the mud. Later, while Private Hawkstone is exploring more of the trench, he is called away by the other soldiers. But as he turns, he notices a body covered in mud and leaned against the wall of the trench. It turns out to be the second German who got away and as Hawkstone calls for help the German jumps at him with a shank and Hawkstone is forced to shoot him. This does not kill him and as they wrestle in the mud, Private Starinski runs up and shoots the German. He falls to his knees and Quinn finishes him with a shot to the head.
The sudden isolation and sinister nature of the trenches soon starts to exact a toll on the British. Plus, one of them, Private Chevasse, is slowly dying from a spinal injury, and the crystal radio they found cuts out after its first legible message, denying them any means to call for support. They all vent their rage upon the German prisoner, who is abused by all but one of them, a 16-year-old boy named Charlie Shakespeare. During their first night, Starinski, is distracted by strange sounds and led deeper into the trenches, where he finds more German dead wrapped in barbed wire, albeit standing up. As he shouts to alert his comrades, one of the corpses suddenly comes to life and ambushes him and later, his comrades find Starinski wrapped in barbed wire, dead. After suspecting the German prisoner, the men violently interrogate him, but Charlie manages to translate his ramblings from French, saying that they will turn on each other and that there's "evil" in the trenches.
Captain Bramwell Jennings, the commander of the battered company, is terrified by hearing strange noises of an incoming infantry charge followed by artillery fire, and runs through the trenches in search of Sergeant Tate, who has sent Hawkstone searching for the captain. More and more frightened, Jennings finally stumbles in the central area of the trench, where the squad has piled the corpses of the many German dead soldiers, and, his nerves on edge, when Hawkstone reaches him from behind, shoots him in the head. Despite that, Tate still fiercely follows orders and refuses to abandon the trench. That same day, the men, believing the attacks and the strange events to be actions done by Germans still hiding in the trench, seals off more passages in an attempt to drive off the enemy.
In the night, more strange noises are heard, and the men's morale and discipline deteriorates even further, with Quinn especially near the border of insanity. Later, after McNess is pursued by an eerie red mist and completely drenched in blood and fear, he runs off, but is shot by a man who turns out to be Private Bradford. A rescue attempt is made by Corporal Fairweather, the company medic, but McNess, crawling across the ground, is pursued by a moving mud mound that drags him underground. Meanwhile, Bradford, who has been found by Shakespeare, is convinced that he is possessed by death, that the whole trench is, and that he is to be killed, urging Shakespeare to do so. When the latter refuses, Bradford runs away.
In the morning, Quinn decides that he has had enough and ties up Friedrich outside the trench and mutilating him, daring any Germans there might be to shoot him and save their comrade. In the trench, Jennings suddenly decides he wants a company inspection, and relieves Tate when the he refuses to comply. Fearing for the outcome of the standoff, both Shakespeare and Fairweather comply with the order, saving Tate from discipline. In the meantime, Jennings notices Friedrich screaming and moves off, going after Quinn. He demands that he too follow orders, but Quinn, already in a murderous frenzy, beats him and stabs him, then confronts Tate, who, though manages to break Quinn's arm, falls into and is hampered by barbed wire and killed by Quinn with a trench club. Quinn then taunts Shakespeare, who holds him at gunpoint, calling him a coward and daring him to shoot. However, barbed wire suddenly appears from out of the ground, wrapping and mauling Quinn, at which point Shakespeare shoots him to end his torture.
Back in the trench, Charlie unties Friedrich, to whom he gives a rifle. Charlie runs off looking for Fairweather. He finds that the paralysed Chevasse, now pale and pestered by flies, appears to be able to move his legs again. However, when Charlie lifts up the blanket that was covering him, he discovers, much to his horror, that rats have been eating Chevasse's legs. Horrified, Chevasse begins to scream uncontrollably, forcing Charlie to shoot him in the head to put him out of his misery. Charlie runs off again and finds the two remaining soldiers, Bradford and Fairweather. Bradford has tied Fairweather up with barbed wire and shoots Fairweather in the head before Charlie can stop him. Charlie finally gives Bradford what he wanted and stabs him in the stomach with his bayonet and then shoots him.
At that point, the soil under the German dead starts to cave in, while barbed wire blocks off every passage. As Charlie tries to escape, he stumbles and is sucked in as well. He wakes up in a dark cave filled with corpses, at which end he finds living versions of the whole group while they were eating just the other night, himself as well. He shouts defiantly that he isn't dead and runs off, reaching the surface of the trenches. Friedrich appears and points his rifle at him. Charlie is exasperated, and tells Friedrich that he helped him. He acknowledges, in perfect English, that indeed only Charlie helped him and that's why he is free to go. Charlie asks what's out there, but Friedrich has already disappeared. Charlie climbs out and leaves to an unknown fate, disappearing into the mist.
Later, another team of British soldiers arrive at the trench and seeing Friedrich they shout at him to surrender and lift his hands. In the last scene, Friedrich gives the camera a knowing stare before the screen fades.
What is the team of British soldiers ordering Friedrich to do?
Surrender