The film starts with scenes of a small village in the Spanish countryside coming together to celebrate the wedding ceremony of two of their own. As the couple and their parents pose for a new family photograph, a long convoy of cars drives into the village, and out steps a Nazi officer whose evil intentions are visible on his face. The officer is offered a glass of local alcohol, which apparently burns his throat as he chokes up, and this causes much laughter among the villagers. The officer immediately orders his men to gun down every single villager and then throw canisters of strange blue-colored smoke into the whole area. As the Nazi army wears protective gas masks, the canisters are thrown toward heaps of Spanish dead bodies.Jan Lozano (Miki Esparbé), a captain of the Spanish army operational in the area, is seen facing the shooting squad before his high-ranking uncle, General Lozano, arrives and stops the punishment act. It is soon revealed that Jan was the captain of an army squad but had been suspended and given the death penalty for head-butting a judge who also happened to have blood ties with the notorious Francisco Franco. It seems that Jan is always up to such strange and reckless behavior, for which his uncle has to often step in and save his neck, but the Spanish general has now lost his patience. In order to get his nephew back on the positive side of the army's opinion, he gives Jan a mission-to deliver a letter inside an envelope to a place named Alarcos. Jan's reaction makes it clear that this is not as simple as it sounds, as Alarcos falls under the Sixth Brigade, which is located on the other side of a stretch of no man's land and enemy territory. Despite Jan's unwillingness to take on the journey because of the many dangers it poses (for he knows he won't possibly be able to return), his uncle explains that the mission is his last chance to avoid the death penalty. Finally, Jan sets out from the Spanish camp along with a young seventeen-year-old boy named Decruz (Manel Llunell), who had been imprisoned for having disobeyed army orders. Since Jan does not know how to drive, Decruz is appointed his driver, and the two head towards their destination in their army jeep. Jan witnesses a Spanish soldier being loaded into a specially marked German train, led by the same Nazi officer who killed the entire village.On the way, Jan and Decruz are stopped by a barricade of German soldiers, and once they are allowed to pass after negotiations, Jan notices that the Germans are putting fences across the valley. Driving on, the two Spanish men see a plane crashing into the nearby jungle and also see the pilot jump out of the plane and land his parachute into the thick forest. Jan decides to go help the pilot, knowing that he is Italian, who was on the winning Spanish Nationalist side in the Civil War, the side that Jan belongs to. When they finally find the pilot, he seems to have died from the fall, as his legs are missing from his knee, and he hangs from one of the trees with the parachute stuck in it. But before the two men can leave the scene, they are taken hostage by a group of Soviet soldiers, who were trying to make their way out of Spain secretly after losing the Civil War, having represented the Republican side. Jan tries to think of some means to escape the situation, but the dead pilot's body suddenly wakes up and bites one of the Russians to death. The strangeness of the matter heightens when the dead Russian now wakes up, having turned into a zombie, and tries to attack all the people. After shooting the zombie dead, the Soviets take Jan and Decruz to their camp nearby, but the place looks massacred with dead bodies lying around. Very soon, though, all these dead bodies rise from the dead and attack the living men and women, irrespective of the uniforms they wear or which side of the war they represent.Understanding the grave danger of the whole situation, the Soviet squad decided to give Jan and Decruz a chance to survive and fight zombies along with them. Although the Soviet leader, Sargento (Luis Callejo), seems to take this decision easily, some of his comrades, including a young woman named Matacuras (Aura Garrido) and a man named Mecha (Álvaro Cervantes), do not agree with this plan initially. Despite their differences, the soldiers from the opposing camps now move together as a unit and reach a safe-house in the middle of the forest. Here they find another team already inside, and this team includes a Spanish Nationalist lieutenant, Jurel (Jesús Carroza), his sharpshooting private Rafir (referred to as Muslim during most of the film, because of the man's religion), and a nun named Sister Flor (María Botto). These three are also already aware of the zombie situation, and they finally decide to join Jan and Sargento. Together they spend some time safe inside the house, and some of them make personal relations with each other. Jan grows romantically interested in Matacuras, but the woman rejects all advances from him and also from Jurel. Decruz recognizes Mecha as a bike racer he admired in his childhood. Sargento develops a soft corner for Sister Flor, who heals a wound on his arm. Jan also spends time with the envelope, now opened, and the letter inside it, and he unknowingly reveals a secret message on the piece of paper. As he burns a side of the letter, a detailed map of the entire region, a valley, that had been drawn with some invisible ink becomes clear.Deep into the night, Rafir shoots an approaching zombie in the head to kill it, only to realize that there are bigger hordes of them coming towards the house. The entire crew now flees from the safe-house, and Mecha, who has a specialty in bombs and explosives, blows up the house with zombies inside before leaving. Walking through the forest, they see electric fences at places, which seem to have been put up to stop zombies from spreading, and that works too, as a few zombies are stuck on these fences, slowly burning themselves to death. Among these is also Brodsky, a heavy-built man who was part of the Soviet unit before he died and turned into a zombie. As Sargento goes through a dilemma over whether to kill his own comrade but only to relieve him of his suffering, Jan takes action and shoots Brodsky in the head.Jan now tells the rest of the team about his discovery-the map, written in German, suggests that the Nazis knew about zombie hordes coming out of the valley, as they had calculated the time and direction these hordes would take, and had therefore put up fences to stop them. With this information, Jan and the Soviets all suspect that the Nazis had some direct involvement in the matter, and their suspicions are soon confirmed. Traveling through the night, the team reaches the now-desolate village that was earlier shown at the beginning of the "Valley of the Dead," and the bride, who was seen earlier, suddenly crawls out of her hiding place inside the church. She reveals all that had happened-the Nazis opened fire on all the villagers, but she was saved by her husband and had only been shot in the leg. Taking shelter in her hiding spot, she saw the Nazis throw canisters of some gas toward the dead bodies, which made all the bodies turn into zombies and come back to life.While all this happened, the Nazi officer and his guards looked on from the bell tower, laughing at the people they had killed and turned into zombies, aimlessly walking around, killing a few of them as entertainment. It is now clear that the Nazis were experimenting with this new drug they had developed, and one soldier of the Soviet camp, a man referred to as Comisario Politico (Dafnis Balduz) in "Valley of the Dead," finds documents about this experiment. This man had already looked suspicious, as he was earlier seen secretly making a telephone call, most possibly to his Soviet superiors, and now he decides to steal these documents and hand over this new bio-weapon to the Soviets. He soon makes his intentions clear to the rest of the group, but the man is swarmed and killed by a few zombies as they make their way towards the church. Jan and Sargento lead their group out of the village through underground tunnels, but lives are lost in the process. The young bride, who was bitten by a zombie earlier or now in a possible confrontation inside the church, turns into a zombie and is killed off by Matacuras. Sister Flor also reveals that she has been bitten, and decides to stay back inside the tunnel to kill off the zombies and die in the process, much to the grief of Sargento. Rafir is also bitten on his foot, but it is found out that the bite had only pierced his boot and had not reached his skin underneath.With very little time on their hand, Jan tries to convince the group to follow him to Las Aguilas, a Nazi-Spanish Nationalist outpost, where he had seen a train with the same symbols that were seen in the Nazi experiment documents. He claims that the train should be the source of this disaster, and, therefore, would definitely have some antidote to reverse the whole process. As the Soviets absolutely reject the idea of walking into the enemy's camp, Jan says that he just wants to end the plight of the common people, already suffering terribly from the civil war, and also reveals that his own brother is a member of the Republican army, contrary to his allegiance. This personal connection seems to make Jan look at soldiers beyond the uniforms that they wear, and the man now convinces his Soviet companions to follow the plan. Together, they all travel to Las Aguilas, but Jan is denied entry by his uncle, who does not want any prisoners to be let inside, and makes his support for the bio-weapon clear. The general tells Jan that the Nationalists plan to kill off all the common people in the valley, whom they have already turned into zombies, by dropping bombs on them from planes very soon. The Nazi officer earlier seen is also present in the camp, and he kills General Lozano, who wants to delay the bombings to protect his nephew.On the other hand, Decruz reveals that he had been bitten by a zombie earlier, and knowing that he was soon about to turn into one, the young man arms himself with bombs all over his body and walks to the Las Aguilas gates. He gets shot, turns into a zombie, and drops the bomb close to the gates to blow it off and kill off guards in the nearby vicinity, thus carving a way for the rest of the group to enter. Jan, Sargento, and the others now enter a tunnel and see the train they had been looking for, with an army of Nazi guards all around. However, the situation soon turns in their favor, though, when hordes of zombies walk into the tunnel, and the Nazis and the zombies face off with each other.As the group now prepares their next plan of action, Jurel and Mecha leave their companions, saying that they do not want to go to such lengths to serve their profession as soldiers, and would rather escape the place. Jan, Sargento, and Matacuras now make their way through the train coaches, hiding from zombies and Nazis, while Rafir provides cover for them from atop a coach. Rafir, though, is soon seen being pulled down by a zombie, while Jurel and Mecha end up inside a car that is quickly surrounded by zombies. Both of them share stories about their respective wives, who were killed by the opposing armies during the war, and then, realizing that the car was filled with explosives, blow themselves off to kill the large horde of zombies. Jan finally finds the coach he had been looking for and enters it with Matacuras, as Sargento goes over to the engine of the train in order to drive it out of the tunnel outpost.Jan and Matacuras now find the Nazi officer, who reveals that it was all his invention and idea to make such a bio-weapon to kill enemies before the rival Soviet forces could make any of it. When asked about the antidote, the officer reveals that there is actually no antidote for the gas, other than a few things that only slow down the process, and the officer cruelly expresses his pride in making such a weapon. It is clear that he has injected himself with large doses of this drug, intending to turn into a very powerful zombie, but the man is shot dead in the head by Matacuras before he turns. Zombies now enter the coach, and one of them bites Jan in his hand. Certain that he too would turn into a zombie, Matacuras very quickly amputates Jan's hand, which soon turns rancid from the bite's effect. The two then take shelter in a small glass compartment at the end of the coach. Sargento manages to set the train into motion but is surrounded by zombies and shoots himself in the head to ensure he does not turn into one. As the train leaves the tunnel, bombs are dropped by planes, killing all zombies, and Jan and Matacuras share a kiss at the same time.After the whole ordeal is over, the two have survived, protected by the compartment, and so has Rafir, who has managed to kill the attacking zombie and now climbs up on top of the coach. Jan and Matacuras are then seen riding on a motorcycle to a Spanish town, where the woman drops Jan off and prepares to leave. Jan asks her to stay back with him as nobody would recognize her as a Soviet, or at least tell him her real name before leaving, but Matacuras denies both his requests as she rides away on the bike with a smile on her face, and Jan then walks towards the town.