An American soldier, Gibson, runs through a Northern European forest. From offscreen, a man tells him to run and hide. He's scared.
Gibson emerges from the forest and comes upon a massive wall with an opening that leads into a maze. Several men are chasing Gibson and are getting closer. Gibson goes into the maze.
Inside the maze, he comes to a four-way intersection. The sounds of other soldiers are getting closer. Gibson runs down the left passage and turns left again at the next intersection.
Six American soldiers emerge from the forest. Perez is on point. Two brothers, Eric and Donnie, come next. They are followed by Thomas, Tarrant and Carter. Thomas is the leader of this squad and he tells them not to rush into the maze. Tarrant is eager to pursue Gibson, who he calls a deserter. Thomas calms him down and laments two missing soldiers Tex and Martini. They say that Martini would've understood the maze, he was the smart one. Thomas blames the maze on "the enemy."
None of the other soldiers has any ideas and Thomas gives up trying to figure it out. He orders Perez to head into the maze first. Carter, then the Grey brothers, go into the maze. Tarrant asks if it's a trap, but Thomas doesn't care anymore and they go into the maze.
Thomas and the soldiers arrive at the first intersection in the maze. He calls for them to split up. He sends Perez and Tarrant down the right passage. Thomas whispers something to Tarrant before they leave. Then he sends the Grey brothers down the middle passage and he heads down the left passage with Carter.
Flashback to a ramshackle shed. Deep in the woods, Thomas and his group of soldier stand outside the shed. Thomas stands outside the door, looking satisfied. He tells Perez that it's his turn. Perez says no. Tarrant gets angry and asks if he's disobeying orders. Donnie and Eric flank Perez menacingly. Thomas gives the order again, Perez says no again. Tarrant joins Donnie and Eric.
Carter and Gibson lock eyes, standing farther away from the shed than the other soldiers. Gibson hints that they should make a run for it. Carter is slow, but he finally understands and says "no." Gibson takes off running. As he goes, Donnie looks up and alerts everyone. Thomas calls Gibson a deserter and he implies that Gibson is going to face violence. Tarrant reinforces Thomas' threat and says that traitors will be killed. Perez and Carter follow the other soldiers in chasing Gibson. Tarrant is crazy as he follows them.
The sun has started to lower and the passages in the maze are covered in shadows. Gibson turns and walks down a long straight passage. He stops to rest and pulls a photo from his pocket. It's him and his wife and child. He tears up, wipes his eyes and puts the picture away. He pulls out a grenade and considers pulling the pin. He puts it away and looks up to see a monster standing at the end of the passage. The creature is covered in long, bristly black hair. Atop his heads are two large horns. He wears torn military fatigues, he used to be human. Gibson looks away as he stands up and when he looks back up, the monster is gone. Gibson grabs his M-16 and quickly moves in the opposite direction from the monster.
Thomas tries to track Gibson, but there are too many boot prints. He guess correctly, though, and he and Carter head in Gibson's direction. They arrive at another split in the passageway. Thomas tries to figure out which way to go when the Monster attacks and slashes his face. Carter escapes, Thomas dies.
Flashback to the shed. Carter asks if he has to go inside. Tarrant and Thomas say he does. Carter goes inside.
Carter runs through the maze until he comes upon Gibson. Gibson aims his M-16 at Carter and tells him to stop. Carter says no, but Gibson aims the weapon ready to fire. Carter stops but says that something's chasing him. The Monster emerges from the shadows and rams its' head into Carter, knocking him against the wall of the maze. Gibson slips into the shadows. Carter sits up and fires at the Monster, but it doesn't matter. Gibson falls on his ass. The Monster kicks Carter's gun away. Then it kicks him in the face. Then it crushes his head. The Monster looks up at Gibson and runs back into the darkness of the maze.
Flashback to the shed. Eric eagerly goes into the shed.
In the present, Eric and Donnie walk down a passage. The Monster leaps out of the shadows and bites Eric on the neck. Donnie fires his M-16, but hits his brother more than the Monster. The Monster roars at him. A dying Eric pleads for help, but Donnie runs away and the Monster takes another bite of Eric.
Flashback to the shed. Thomas tells Tarrant to go inside. He thanks Thomas and tells them he's going to show them how it's done.
Back in the maze, Perez and Tarrant come to a split in the passage. Tarrant insults Perez as he tries to figure out which way they should go. Tarrant heads down the passage that leads towards the center of the maze. He continues to berate Perez as he heads down the passage when he realizes that Perez is gone. He doubles back to go find Perez and kill him as a "deserter." He randomly picks a direction and while he's pointing that way, the Monster bites his hand off. Tarrant pulls out a knife and stabs the monster, but the knife breaks. It punches him in the chest, crunching his bones. Then the Monster bites his face.
Flashback to the shed. It's Donnie's turn to go into the shed and he's anxious to go in.
Gibson walks through the maze and arrives at a large open area. Donnie runs into the open area from another passage. Gibson aims his weapon at Donnie, but he's frightened. He tells Gibson something killed Eric. Gibson tells Donnie to drop his weapon. He does.
Perez emerges from the third passage and asks them what's going on. Gibson turns his weapon on Perez, who doesn't flinch. Perez says he has no problem with Gibson and they relax. Donnie asks for his gun back. Gibson has seen the Monster, so he allows it.
The Monster emerges from the fourth passage and roars. Donnie scrambles for his gun. The three men unload their clips at the Monster, but it stands unharmed.
Perez rushes the Monster with his knife, but it swats him to the ground and he loses the blade.
Donnie pulls out his sidearm and aims it at the Monster. He walks closer and closer, shooting it as he goes. Once he gets close enough, the Monster disembowels him with a swipe of his claw.
Perez grabs his knife. He tells Gibson to use his grenade on the Monster, but he says it won't work. Gibson tosses his gun to the ground.
The Monster gestures that Gibson and Perez should fight each other. It threatens them and they face off. Gibson pulls out his knife and rushes Perez. Perez dodges. Gibson attacks again, but misses. Perez slices his arm with a counter-attack. Gibson taunts Perez, but Perez doesn't bite. Gibson rushes Perez again and Perez ducks low and slices Gibson on the leg. He falls to the ground, pretending to be in pain, but he sweeps his leg and Perez falls to the ground. They tangle up and roll around. Perez smashes Gibson in the face with the butt of his knife. He forces Gibson to drop his knife.
Perez stands up and kicks Gibson's knife away. He points the knife at Gibson and asks the Monster if that's good enough. Gibson sneaks out his grenade.
Flashback to the shed. Thomas ask Gibson if he's ready to go in the shed. He says he is. He does.
Perez asks the Monster what it wants. Gibson pulls the pin from the grenade. The Monster rushes at Perez. Perez drops low and the Monster leaps over him. The grenade bounces off of the Monster and lands in Gibson's lap. The Monster covers Gibson's body as the grenade goes off, fully forcing all the blast towards Gibson who is destroyed in the explosion.
Perez stands and looks towards the Monster who gets up. The Monster roars at Perez and then goes back into the maze.
Later Perez exits the maze and starts to follow a path down into a valley and towards a village.