Juan Oliver wants to make a good impression at his new job as a prison officer and reports to work a day early, leaving his pregnant wife, Elena, at home. During his tour of the prison, an accident occurs that knocks him unconscious. He is rushed to the empty but visibly haunted walls of cell 211. As this diversion unfolds, convicts break free and hijack control of the penitentiary. Aware of the violence that is to come, the prison officers flee, leaving Juan stranded and unconscious in the heart of the riot. When Juan awakens, he immediately takes stock of the situation; in order to survive, he must pretend to be a prisoner.
Juan manages to convince the other prisoners that he is one of them, and that he just entered the prison that very day for homicide. He not only makes himself believed as inmate, but befriends the violent, deep voiced leader of the riot, Malamadre, who takes him under his wing.
Malamadre discovers that Basque terrorists associated with ETA are being held in the same prison, and plans to use them as a bargaining tool. When this hits the news, it triggers a stream of prison riots throughout the country as well as demonstrations in the Basque Country. Juan quickly realizes that he is on his own as the situation becomes more politicized and the government gets involved, afraid of potential ramifications in the Basque community.
Juan’s pregnant wife, Elena, slowly learns about his impossible situation and heads for the jail. Once there she is brutally beaten outside the prisons, while trying to get information about her husband. A TV inside the prison cell block reveals the information to the prisoners. Camera footage indicates that Elena was at the riot. Fearful for her safety, Juan insists on learning information regarding her status.
Malamadre demands information of all those injured by the riot. The authorities respond with a short list of only four names. This infuriates the prisoners, who believe that the police are withholding information, and decide to kill one of the ETA prisoners. However, Juan intervenes and suggest they merely cut off an ear, an act he is forced to perform himself.
The authorities then provide additional information on all the injured. Juan learns Elena is in the hospital, though he is told she is recovering. Juan insists on talking to her but is rebuffed. Utrilla, the police guard who hit Elena, is suspended and asked to leave. Meanwhile, Apache, an inmate close to Malamadre, learns the truth behind Juan’s status as a guard, having contacted a friend at the police via a secret cell phone. He tells Juan he will give him a chance to defend himself before letting Malamadre know.
Later, Malamadre has Apache tell Juan that he needs to speak to him. Juan is brought to a secluded room, the door closed behind him, and several of Malamadre’s men surround him. He asks, nervously, “What faces?” in an effort to defuse the situation given his worry that Apache betrayed his true status. Instead, Malamadre shows Juan a cell phone video from a different camera at the outside riot. The footage shows that Elena was beaten by Utrilla, another prison guard, who then proceeded to leave her on the ground. Juan insists that Utrilla be brought inside.
Inside, Utrilla is faced with the cell phone footage by an infuriated Juan. Juan again insists on talking to Elena, but he instead learns that both she and her unborn child have died. In response, the prisoners viciously beat Utrilla until he loudly insists that Juan is a prison guard. He asks Juan to tell the truth; instead, Juan slits Utrilla’s throat and fully joins the prisoners in their fight, having lost all will and reason for life outside. In private Malamadre and his other cohorts are furious but Malamadre leaves him alone, given that he ruthlessly killed Utrilla. Juan proceeds to his cell and attempts to commit suicide by hanging himself with a belt. He fails as the belt breaks under his weight.
Malamadre receives a call from the negotiator asking him to collect a file. They tell him that Juan Oliver is a prison guard and leave the file for Malamadre to see. Malamadre says it is only a piece of paper and heads back inside telling the negotiator that they are just trying to make him kill Juan.
The government sends a messenger to deal directly. The government acquiesces to Malamadre’s demands. However, Juan drags Utrilla’s corpse in front of the liaison and insists that the government agree in public or they will kill the ETA prisoners. Malamadre is furious at being upstaged though he understands that Juan is correct. Malamadre tells Juan that if the GEO team invades instead of agreeing, then he will either die or kill Juan. In either event, Malamadre insists only one of them will live, as punishment for Juan’s betrayal.
The government does not agree to the demands and sends in the GEO team. Apache kills Juan and severely injures Malamadre, and it is unclear if he will become the new prison king or if Malamadre, as soon as he will heal from his injuries, will exact vengeance upon him. At a secret inquiry held by the government, the prison officials regret Juan’s death but did the best they could in the situation. Finally, the prison official simply asks, “Any more questions?”