Deathwatch beetle (Xestobium refuvillosum)
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The story is set during World War I. In the chaos of a trench warfare battle, a small group of British soldiers get lost, surrounded by a thick mysterious fog which is not a poison gas. As they emerge from it, the British come across a complex network of deserted German trenches. Convinced that they have broken through the enemy lines, the soldiers decide to secure. They find several bodies wrapped in barbed wire, and they realise they are perhaps not alone, though there is a great deal of confusion among them concerning why the Germans seem to have killed each other.
One of the men in the British squad has been paralysed due to a spinal injury and is laid under a blanket. The group capture a German soldier who is abused by all but one of the soldiers, 16-year old Charlie Shakespeare. During their first night, one of the men, Starinski is distracted by a strange sound and led to more dead bodies wrapped in barbed wire, but mysteriously standing up. As he shouts his comrades, one of the corpses ambushes him and later, his comrades find Starinski painfully 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 is evil in the trench.
Capt. Bramwell Jennings, who appears to lose his head early on, gets lost and staggers around the trench looking for Hawkstone. The strange noises he hears make him more and more nervous until, when he finds Hawkstone, he is surprised and shoots him in the head. Even after this, the men follow his command and stay a night in the trench (most opt to leave, but the sergeant follows rules fiercely and says they must obey their commanding officer with the threat of a gunshot to the head in case of leaving).
In the night, strange noises are heard, and one possible explanation to what is going on is put forward when Quinn, whose sanity is slowly disappearing, says "You little Hun bastards, what've you been doing here?" implying that the Germans had been running strange experiments there. Later, after Pvt. Willie McNess is pursued by red mist and completely drenched in blood and fear, he loses his nerve and runs off, but is gunned down by an unseen enemy who turns out to be Anthony Bradford. A rescue attempt is made by Doc Fairweather, but McNess is dragged under the ground by a mysterious moving mound. Meanwhile, Bradford is convinced that death is inside him and insists that Charlie killed him.
In the morning, Quinn decides that he has had enough and ties up the German prisoner outside the trench and starts beating him, tempting any Germans there might be to shoot him and save their comrade. In the trench, Jennings suddenly decides he wants a company inspection, but the sergeant tells him there is barely any of the company left. The two get into a fight and Jennings is about to shoot him when two of the men humour him to save Sgt. Tate's life and line up for inspection.
Jennings demands that Quinn also line up but is told that he is outside the trench, so he goes after him. Quinn kills the captain by stabbing him repeatedly and then the sergeant by beating him horribly on the head (who had tried to rescue Jennings) but, when held at gunpoint by Charlie, Quinn is bound and savaged by barbed wire that reaches out of the ground. Charlie ends his torture by shooting him.
Charlie unties the German, who he gives a rifle in a moment of trust. Charlie runs off looking for the three remaining men. He finds the paralysed soldier, appears to be able to move his legs again. However, when Charlie lifts up the blanket, he his horrified to see that rats have been eating the soldier's legs, who then 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 Doc Fairweather. Bradford has tied Doc up in wire and shoots him 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 fires.
At that point, a large sinkhole appears in the ground and barbed wire bars all the exits, trapping Charlie. The mud starts sucking everything into it, including Charlie. Charlie tries to climb out of the trench but falls into the sinkhole. Charlie wakes up in a dark cave filled with living versions of the whole company. He shouts that he isn't dead and runs off, reaching the surface of the trenches. The German soldier appears and points his rifle at him. Charlie is exasperated, and tells the German that he helped him. The German acknowledges, in perfect English, that only Charlie helped him and tells him that's why he is free to go. Charlie asks what's out there, but the German has already disappeared. Charlie climbs out over the top and leaves to an unknown fate.
Later, another team of soldiers march up to the trench, different from the first group (the films protragonists) but with close similarites in their actions. They spot the German in the trench and shout at him to put his hands up. The German gives the camera a knowing look, and the film ends.