Juan Oliver reports to work as a prison guard a day early, and what starts as a routine tour of the facility quickly spirals into a crisis. A falling projectile catches him off guard and he is knocked unconscious, only to be rushed to Cell 211 as chaos erupts around him. The incident is later revealed to be a calculated diversion designed to blood the prison with a full-blown riot, allowing convicts to seize control while his colleagues flee, leaving him stranded inside the cell.
When he awakens amid the upheaval, Juan pretends to be a newly transferred inmate convicted of homicide. He is brought before the riot’s formidable leader, Malamadre, who begins to mentor him after Juan suggests tactical tweaks to the uprising that could give the inmates an edge in negotiations. Yet his real aim is more strategic: to maintain lines of communication with the remaining guards and with an incoming GEO team that is preparing to storm the prison.
[Malamadre] discovers that Basque terrorists connected to ETA are detained in the facility and tries to weaponize them as a political bargaining chip during the negotiations. News of the riot spreads beyond the walls, sparking a wave of protests in the Basque Country. Fearing the political fallout, Spanish ministers hesitate to authorize a direct intervention, which leaves Juan isolated inside the burning prison.
Elena, Juan’s pregnant wife, learns about the crisis and heads toward the prison. There, protesters clash with police, and Elena is struck by an officer. A television screen inside the prison broadcasts the unfolding events, confirming her presence at the protest. Desperate to reach Elena, Juan attempts to maintain contact with the guards, while Malamadre presses for information about the injured. The authorities supply a list of four names, and the escalating pressure drives Malamadre to decide to kill one of the ETA prisoners.
Juan intervenes and proposes a brutal measure to force a response, a decision he is compelled to carry out: a severing of an ear. The authorities later inform him that Elena is in the hospital, slowly recovering, but Juan’s request to speak with her is rebuffed. Utrilla, the police guard who struck Elena, is suspended as investigations begin.
Apache, an inmate close to Malamadre, uncovers the truth about Juan through a smuggled cell phone and offers him a slim chance to defend himself before Malamadre learns the full story. Later, Malamadre has Apache bring Juan to a confrontation and shows him footage of Elena being beaten. Juan begs to speak to Elena, only to learn that she and their unborn child have died. In retaliation, the inmates brutalize Utrilla until he reveals that Juan is a prison guard. Instead of exposing the truth, Juan slits Utrilla’s throat.
Malamadre resists the urge to kill Juan, following a belief that public executions could alienate the inmates. A government negotiator later provides Juan’s employment records to verify the guard’s identity and asks Malamadre to cooperate in exchange for concessions, but the leader still refuses to kill him.
As talks inch forward, the GEO team finally moves in to quell the riot. Juan drags Utrilla’s corpse before the government liaison and demands a public acknowledgment, threatening to execute the ETA prisoners if the government does not concede. Malamadre warns that if the GEO intervention occurs instead of a public agreement, he risks death or betraying Juan. When the riot is eventually suppressed, Apache kills Juan, and severely injures Malamadre, who then shifts his stance and begins working with the government against Apache.
