Juan Oliver is about to become a prison officer. On the first day of his new job, two colleagues show him around the old prison, telling him about the rough work and about Apache, a Russian informer. All of a sudden, and due to some building work, some plaster falls from the ceiling and hits Juan Olivier, who passes out. The guards take him into cell 211, which is empty, to try and revive him. But, as luck would have it, that's when a riot breaks out at the high security FIES cell block, the one that houses the most dangerous inmates.
Juan's colleagues run away, leaving him stranded. When he wakes up in the cell he takes stock of the situation: if he wants to save his skin, he must pretend to be a prisoner.