GP: Guard Post is on the frontline inside the Demilitarized Zone (DMZ). The tour of duty lasts for 3 months and nobody is allowed to leave. A maze-like concrete bunker is approximately 990-1650m2. Soldiers are fully armed and ready to engage at all times. Guard Posts are a vestige of the Cold War.
The date is May 29th 2007, 8.03pm. A group of soldiers from the military police (MP) break into one of the guard posts, numbered GP 506. They search from room to room, looking for something. Eventually, they enter a large, dark room and notice numerous bloodstains awash on the floor and walls. One of the MP soldiers hit something on the floor and find a corpse. The rest of the soldiers spot someone under their torch lights, to reveal a half-naked blood-drenched man breathing heavily, and holding a large axe. The man starts to laugh, and the MP soldiers move in to disarm and put him into custody.
At the same time, Sergeant Major Seong-gyu Noh, along with a friend in the army, are paying their last respects to Noh's wife. Later, in their car, Nohs friend informs him of an assignment that the military has ordered him to do: head to Guard Post 506 to investigate a strange incident there. Noh learns that the guard post is headed by the army chiefs son, 1st Lieutenant Jeong-u Yoo. His friend warns him that this assignment may very well cost him his job, if hes not careful in handling the situation. He continues by telling Noh that all the soldiers at GP 506 were killed except for one (the man with the axe), whos currently in a coma, and that hes the only suspect. Noh has until 6.00am to investigate the cause before more soldiers from HQ will come and clean up everything.
Noh arrives at GP 506 about an hour later. On the way to the office, he passes a boiler room that has words written in blood on the wall, which says: Kill all of them. He is brought to the recreation room where the bodies were found, and watches in shock at the amount of blood splattered around the room. Noh is brought to the unidentified man at the beginning, who is comatose. The MPs are going to bring him to a military hospital. Noh arrives in the guard post office, where he meets 1st Lt. Bang, the one in charge of bringing the soldiers here, and the medic, only known as Doc. He tells both of them that he will be assuming command of this investigation from this point onward.
Heading back to the recreation room, Noh starts searching for clues, and finds a battered birthday cake in blood. He finds a soldiers dog tag inside it. Comparing the name incripted on the tag to the lockers, he identifies the comatose man, who is Corporal Jin-won Kang. Later, he and a soldier, Sergeant Yoon, comb through the maze-like tunnels that make up the bunker. Doc, who is preparing the bodies to be shipped to a military coroner, calls Nohs attention to an apparently missing soldier. The 19 bodies they found and Cpl Kang, do not tally with the 21 soldiers who are stationed at GP 506. Noh wants Doc to check again, only to reconfirm 19 bodies. Noh then allows Doc and a group of soldiers to transport the bodies and Kang away from the GP. Meanwhile, 2 MPs are struggling with the guard dogs stationed there. The guard dogs has gone fierce and will not allow the MPs to handle them. One of the dogs bite a soldier in the arm.
Flashback: Private Cho, one of the soldiers at GP 506, is sent by his superior at night to check up on the guard dogs incessant barking. Cho followed the noise of the dogs barking into a small alley, only to find the noise ceased. Cho called the dogs name and rounded the corner, only to witness in shock at one of his superiors, Sergeant Ma, his face drenched in blood, who is busily biting the dog to death.
In the present, Noh checks the guard posts records and notices that someone has been controlling the weapons lately. All personal weapons were kept in the armory and not in the barracks, which was unusual. One of the soldiers reports to Noh, informing him that they have found another survivor, inside the generator room. Noh manages to disarm the man, who is wielding a knife. Later, the man eats hungrily at food given to him by the MPs, as Noh questions him. The man says he is 1st Lt. Yoo, the son of the army chief and the head of GP 506. However, he refuses to say anything more, insisting on the MPs sending him back to base HQ.
Noh reports to his superior on the phone, who wants him to bring Yoo back, despite his objections as the investigation has not been completed yet and will be forced to end once the clean up crew arrives in the morning. Noh refuses to listen anymore to his superior and ends the call. He then goes off to look for Yoo at the canteen, only to find hes not there. One of the MPs tells him that he went to wear his uniform. Noh immediately rushes to the office, to find Yoo tearing pages off his own journal. Noh pulls him off and puts him back in custody at the generator room. However, he finally decides to let Yoo leave GP 506 and puts him in a jeep with another soldier.
Sergeant Yoon finds a video-cam stashed inside the food storage room. Noh reviews the contents. It shows the soldiers of 506 celebrating someones birthday, and suddenly the recording is written over, to show Cpl. Kang. With strangely determined eyes, he announces his intention to kill all his mates in the platoon and he has to end this himself. However, the recording fizzles and Noh is unable to see more. Meanwhile, Doc and the group of soldiers transporting the bodies are blocked by a landslide and have to turn back.
Flashback: Cpl. Kang brings some of his men to the machine-gun at the top of the guard post and fools around, accidentally causing the gun to fire and the North Korean soldiers stationed 300 meters to fire back. After this, Cpl. Kang and the rest of the platoon are punished, with Kang himself having to do chores for the rest.
In the present, Noh reads Kangs personal records that he was a troublemaker, but he thinks that Kang doesnt look like one who would go insane and embark on a killing spree. He pores over Yoos journal. Flashback: Yoo questioned one of his sergeants on the disappearance of Sergeant Ma and his men, who went out into the woods one day and gone missing, only to stagger back to 506 looking sick. Private Kim found Ma drooling at his post, and Ma walked eerily towards him. In his fear, Kim ran and fell down the stairs injured. However, he did not remember exactly what happened. Kim, Ma and his men had the same symptoms, scars on their bodies, having no memory what happened to them. As Yoo decided to lock up Ma and his men, one of the soldiers informed him that Kim was found dead with inflicted wounds all over his body. The rest were shaken over Kims death. Cpl. Kang went on sentry and overheard Yoo assuring HQ on the phone that everything was fine.
In the present, Doc returns with the bodies and Cpl. Kang lapses into critical condition. Despite Docs attempts to resuscitate him, Kang dies. Later, the jeep containing Yoo returns and he refuses to leave the vehicle, banging his head on the window. Noh drags him out and back to 506. Yoo later tells Noh and Yoon that after he locked up Ma and his men, it wasnt the end of the problem. Private Cho was found breaking his hand with a wrench after being ordered to fix the shower room pipes. Yoo talked to Kim again about the incident, but he still remembered nothing. As they talk, Kim supposedly saw the dark silhouette of what he thinks is Sergeant Ma walking towards the medic room where they were. He went hysterical and pointed the shadow out to Yoo. Yoo turned around to face the shadow, only to black out for a few minutes. When he was woken by the medic, Sergeant Kwon, he found that Kim was dead on his bed. After the incident, he ordered the confiscation of live ammo from his men.
In the present, some MPs go sick from the rain pouring outside. Doc is unable to find any cold medicine, which supposedly should have sufficient amounts of it. Meanwhile, Cpl. Kwak and his buddy are sent for sentry duty. He finds a dark room. Suddenly the door closes behind him, and loud banging sounds come towards him. Sergeant Yoon washes his hands in the toilet and finds strange rashes on them. He hears a sound in one of the cubicles and investigates. He discovers Cpl. Kwak, who is supposed to be on sentry, bashing his platoon mate to death with his rifle butt. He goes off to inform Noh, who along with other men rush to the toilet, only to find the dead soldier and no sign of Kwak. They find Kwak in the room he first went in. When they shine their torches on him, Kwaks clothes are found with blood, with him feeling confused. He is arrested and interrogated, but he claims that he does not remember anything. Doc arrives to check on Kwaks neck, finding the rashes.
Flashback: Sergeant Ju and his men broke into the radio room, armed and threatening to kill anyone who enter. The group said that 1st Lt. Yoo lied to HQ about the situation in 506. Yoo, Cpl. Kang and another sergeant went in to defuse the situation, but is unable to convince the group to drop their weapons. They struggle with the men and one of them fires his rifle, killing the sergeant. Yoo ordered Sergeant Ju and his group to be arrested, but by now most of his men, including Kang, having enough of it, refused his orders. They instead locked him up inside the generator room. Later, the men intended to call HQ, but the radio was broken during the gunfight. Sergeant Kwon found that the doors leading out of the bunker are locked, and asks Yoo in the generator room at gunpoint, where the keys are. Yoo refuses and both of them struggle, leading to Yoo killing Kwon by shooting him in the head.
In the present, Yoo tells Noh about all this, and Noh asks him if there is a disease going around. However, Yoo shook his head. Doc inspects the bodies and finds rashes on their chests. He summons Cpl. Kwak and tears open his shirt, only to find the same rashes within. Later, Noh and Doc discuss the same symptoms present in the other bodies and some of their men. In a matter of hours, the disease that inflicted the men of 506 will be present in their men as well. Doc surmises that a rabic-like virus infects the men, making them turn violent and kill each other, but he does not know how the infection spreads. Whatever is out there in the woods was left untouched for 50 years, due to North-South Korea confrontations over it. Noh wants to know why Cpl. Kang killed the rest of his mates, but Doc says that is not important, as the disease may be spreading as they speak. Later Noh examines the video recording again, and searches through the 506 mens belongings. He discovers a photobook and as he looks through it, he finds something important and rushes to look for Yoo, but hes missing. At the same time, Yoo has sneaked out of 506, and unbeknownst to the driver and sentry, enters the ambulance to look at the dead Cpl. Kang.
Flashback: The truth is revealed, 1st Lt. Yoo was actually part of Sergeant Ma and his mens group when they went into the woods. He was also sick along with the rest when they came back to base. When Kim saw the dark shadow, it was fact an illusion and Yoo was the one who suddenly spasmed while questioning Kim and beat him to death with the IV drip pole. The medic, Sergeant Kwon, found him drenched in blood. Later, Kwon advised him to contact HQ and tell them about the situation, but Yoo, who did not want to go down for killing someone, refused to accept and thought that he could fix this situation. He intended to rewrite the duty logs of what happened, and told Kwon to dispose his bloody clothes without anyone knowing. After making his report to HQ, Yoo turned to Kwon, whose face is the man Noh and the others found in the generator room. Only after seeing the photobook, Noh realizes the man they thought was 1st Lt. Yoo wasn't, he was in fact Sergeant Kwon.
In the present, the man who is Sergeant Kwon sneaks out of the ambulance and is discovered by the sentry. He gets into a struggle with him and breaks the sentrys neck. As Kwon picks up the sentrys rifle, the sentrys face starts to bloat in front of his eyes and he makes croaking noises. Kwon, unable to shoot him with the rifle, takes out his knife and stabs him in the hand. When the ambulance driver tells Noh of Kwons presence and murder, he along with his men rush outside, only to see Kwon trying to escape using the ambulance. He runs over the sentrys body. Noh runs after him and fires his gun, managing to make Kwon lose control and crash into the side of the bunker. Noh drags him out of the vehicle. As this is going on, the dead sentry suddenly comes back to life and stabs one of the soldiers standing near his body. Noh takes one of his mens rifles, shoots the seemingly undead corpse and finally makes him stop moving. Kwon is then locked and tied back up in the generator room.
Flashback: After locking Yoo up, Kwon is unable to tell the 506 men about their symptoms they are having, despite being the medic and having to keep the truth about Yoo to the men. Later when he hid in the room where the infected men were kept while Kang chased him and demanded an answer, he suffocated one of the infected, Hoon, to death to end his suffering.
In the present, Noh, Doc, 1st Lt. Bang and the sergeants gather the remaining men for an inspection due to the disease. Doc divides up the men according to whether they have the rashes on their bodies. However, when Noh orders the infected to strip their weapons and hand them over, they refuse and the soldiers present have a standoff. Meanwhile, Kwon, tied up in the generator room, struggles with his bonds and knocks out the electric supply temporarily, blacking out the bunker. This causes the infected men to fire out of fear, killing Bang and most of the non-infected soldiers. Later, Doc checks up Kwak, only to find the rashes gone. Noh goes to Kwons cell and demands to know everything.
Flashback: Yoo learnt that Kwon wanted to revolt his command by telling HQ all of what has happened. He threatened to blame Kwon that all the deaths that happened was due to Kwons mutiny, but the latter is unfazed. He instead blamed Yoo for going into the woods and bringing back the disease, turning them into monsters. He intends to survive, no matter the cost. Later, a soldier discovers a corpse in Yoos cell and thinks it was Yoo who killed Kwon and escaped, due to the corpses face battered beyond recognition and his clothes belonged to Kwons. After the soldier left, Kwon hides in a dark corner without his uniform, muttering that he will survive. He hid until the MPs from Nohs command finds him down there. He assumes Yoos identity to Noh and his men.
Noh, upon learning the truth from Kwon, beats him up in anger, due to him withholding the truth and his own selfish reasons of wanting to live, have caused his own men and Noh's group to die. Kwon retorts back, saying that HQ did not care about them when they were dying like mad dogs here. Later, while staring at the rain outside, Noh touches his family photo and comes to a terrible resolution. He goes into the office and finds the keys to the bunker. When leaving, he hears sobbing sounds from a corner, to find Sergeant Yoon. He discovers Yoon is infected. He embraces the crying Yoon and unknown to him, points a gun to his head. Yoon, realizing what he is about to do, struggles from his grasp. Noh shoots him dead and slumps himself in a corner. After he recovers, Noh goes into the armory and collects weapons, while Doc asks him if he wants to report to HQ about this. Noh, not answering him, instead asks if theres a cure to the disease. Doc does not know, the only thing he understands is that the virus would not survive in open air without a host, and it will die if the host dies too. Noh says that if they all die, the virus will die with them too. Doc understands what he is going to do and tries to stop him, but Noh is adamant in his decision. He now understands why Cpl. Kang had to kill all his mates. He checks Docs hand, to find it covered in rashes. He then leaves Doc without a word.
Meanwhile, the infected men have decided to deny the virus ever existed, and wants to leave with the soldiers that will arrive to clean up at 7.00am. They hide their rashes under camo paint. Sergeant Lee, one of the infected, organizes the men and prepares to leave, no matter the consequences. Kwon wakes up in his cell and finds a supposedly dead Kang approaching him with his axe. However, it turns out to be Noh, who shoots him dead. The infected soldiers find the door leading outside the bunker put under heavy locks. Another soldier reports that both Yoon and Kwak were found dead with a gunshot wound (killed by Noh). Lee with the soldiers intend to fabricate a report for HQ, that Kang killed his men and later Yoo went insane and killed their own men too, and that theres no mysterious disease. Lee orders the men to look for Noh, who is busy spilling kerosene all around the rooms, passageways and corridors of the bunker. Lee and the soldiers manage to corner Noh in the shower room. Unable to kill Noh, Lee wants him to join them and deny that the disease ever happened, but Noh refuses. Doc enters the room with a rifle, having decided to aid Noh. Lee demands the keys to the bunker but Noh refuses to give them.
One of the soldiers goes hysterical and fires at Noh and the gunfight ensues among the men. Noh and Doc manages to kill 3 of the men, with Noh injured. Doc runs out of ammo in the end and is shot to death by Lee. Noh retaliates by killing Lee. However, the only soldier left, a private fires at him. Noh collapses on the ground, coughing out blood, and then dies. The private grabs the keys from his dead body and staggers all the way to the entrance. Outside, new soldiers sent from HQ arrive to meet up with Noh and his men. The private fumbles with the keys and opens the door to the bunker, thinking that he can finally get out, only to watch in horror as a tripwire breaks when the door is opened, which is connected to explosives. The blast kills the private instantly, and the explosion sparks the kerosene inside the bunker, washing it aflame. The soldiers approaching the bunker instantly drop to the ground for safety, while GP 506 has turned into a smoking ruin.
The final scene shows Cpl. Kang in the video-cam recording as it was before it became spoilt. Im Specialist Kang of GP 506. We are all infected with a mysterious disease. This is the sign. He shows the cam the rashes on his chest. It makes you violent and attack other people. After a while, it becomes dormant, but its more dangerous. Im putting an end to this. By the time this tape is found, we should all be dead. If you find any survivorskill them. He is later shown holding a birthday cake and carrying a bag of weapons, silently walking along the corridor to the rec room. His other platoon mates are heard wondering why he asked them all to gather. As he enters the room, his mates tell him that they dont have time for this. Gunfire suddenly ensues in the room and screams are heard, and the film ends.