On the Western Front in 1918, Lieutenant Berton, Rossif Sutherland, a Canadian tunneller, digs 78 feet (24 m) beneath a German position to rescue fellow tunnellers. He accidentally startles a survivor who screams, alerting the Germans above. In retaliation, the Germans detonate charges over Berton’s shaft, trapping him underground for twelve days while the pressure and fear mount around him.
Three months later, on November 2, 1918, a pair of British Army officers from the Intelligence Corps, Major Jennings Ted Atherton and Dr. Priest Charlie Carrick, debrief Colonel Ashcroft, John B. Lowe, with information about a deep underground German bunker hidden in the Argonne Forest, known as Trench 11. The base is commanded by Reiner Robert Stadlober, a strategist who specializes in developing dangerous chemical and biological weapons. Jennings and Dr. Priest believe Trench 11—named for its distance behind enemy lines—is being used as a launchpad for Reiner’s experiments, and they request a closer look since the Germans failed to destroy it during their retreat. Because the Argonne lies in the American sector, Colonel Ashcroft arranges a U.S. escort and allows Berton to return from leave to assist with the investigation. During his leave, Berton forms a quiet romance with a French woman, Veronique Karine Vanasse, though duty soon pulls him away again.
Berton joins Jennings and Dr. Priest on an expedition to Trench 11, along with their American escort: Captain Cooper Luke Humphrey, Sergeant Pronger Jeff Strome, and Private Kelly Adam Hurtig. They find the trenches littered with dead German soldiers and learn that the Germans had shelled the area before the Allied arrival. Back at German headquarters, the 5th Army leadership orders Reiner and Capt. Mueller, a hardened officer Shaun Benson, to erase all evidence of the bunker, calling it the Wotan Complex so the Allies cannot uncover the truth about the weapons program. Mueller, who is disgusted by Reiner’s work, is tasked with escorting him to the bunker to complete the cleanup.
In the trench, the group uncovers signs of German infighting and sabotage. They descend into the bunker and encounter a sealed door and a German soldier left behind who begs them not to open it. Ignoring him, they move deeper and are attacked by crazed enemies who vomit and clash with their defenses. Berton and Cooper are separated and forced to improvise when the corridor collapses above them. Meanwhile, in a hospital-like chamber, Pronger is attacked by a crazed man, and they soon discover a worm crawling from a gunshot wound. Dr. Priest conducts an autopsy on the corpse and confirms that the worms are not natural but engineered. He concludes the parasite is designed to spread violence, not just infect.
A mutiny erupts as Pronger and Kelly push for escape, clashing with Jennings. The group is then overwhelmed by German soldiers who kill Pronger and Jennings and seize Kelly and Dr. Priest. Berton and Cooper manage to find an exit but are captured along with the rest. Mueller presses forward, but Dr. Priest refuses to cooperate with Reiner’s plans. The German mastermind keeps a close watch as Reiner’s interrogation tightens, probing Dr. Priest’s understanding of the biology behind the worms. Priest realizes the worms were designed to target livestock in Allied nations and then jump to humans; Reiner reveals his grander scheme to weaponize the organism across Europe to rebuild it under German rule. He tortures Priest as part of his insistence.
Mueller and Berton locate the charges and learn that the explosives were never properly rigged in the first place. After Reiner executes Dr. Priest, the plan to flood the lower levels with poison gas commences and the remaining parasites are gathered. Mueller and Berton race to detonate what remains; Kelly’s condition worsens and he dies after killing a comrade—Berton is forced to shoot him to prevent further disaster. Mueller tries to convince Berton to stand down so the parasite cannot spread, but Berton holds firm, aware that the weapon could threaten everyone if it escapes. In the meantime, Reiner intensifies his efforts, torturing Priest as he presses for loyalty to the broader mission.
With the clock ticking, Mueller and Berton locate the actual detonators. A desperate struggle ensues as Berton arms a triggering mechanism, while Mueller sacrifices himself to give Berton a chance to escape. Reiner confronts Berton in a brutal showdown; the German commander fights fiercely, even as Berton fights for his life. Reiner’s eye is finally claimed by a vial he wields in his own hand when Berton interrupts his escape. The tunnel machinery collapses around them, and Berton crawls free, though he is badly wounded and limps away from the site. As he staggers toward the daylight, he is haunted by a vision of Veronique, leaving his ultimate fate unresolved and keeping the focus on the haunting consequences of the buried war crimes unearthed at Trench 11.
