An injured man finds himself being chased through the woods at night, ultimately succumbing to his pursuer. As he refuses to reveal the whereabouts of a missing woman, he is buried alive by his attacker.
Police officer Martín Salas embarks on his first night shift driving a prisoner transport vehicle, paired with Officer Montesinos. Tensions rise when Montesinos chooses to humiliate some prisoners during their transfer, prompting Martín to step in and diffuse the situation to prevent any escalation into violence.
However, as they navigate through thick fog, they lose both sight and radio communication with the accompanying police car. Suddenly, the transport vehicle comes to a jarring halt due to a spiked strip laid across the road. Montesinos exits the vehicle to investigate but fails to return. Upon Martín's search for his partner, he discovers a harrowing scene: Montesinos appears dead, and the police car lies overturned with the officers lifeless inside.
A sniper begins to shoot at Martín, and though he fights back, he sustains a gunshot wound to his leg. Seeking refuge inside the transport vehicle's prisoner compartment, he finds himself in a dire situation when one of the prisoners attempts to overpower him. He manages to defend himself with his weapon, but not before gasoline is poured into two cells through the ventilation system and ignited, leaving Martín with no choice but to open the cells to rescue the inmates, though one prisoner tragically succumbs to the flames.
The freed prisoners soon hear an ominous message through the intercom, claiming that Martín's gun is empty, urging them to escape. Chaos ensues as prisoner Nano violently asserts dominance, killing another inmate, insisting he orchestrated the hijacking and seizing Martín's key. The hijacker, Miguel García, commandeers the transport vehicle, threatening to execute everyone if they do not hand over Nano. A tense struggle erupts over the key, ultimately leading Nano to swallow it. Martín is captured and imprisoned alongside Nano as the other prisoners scramble to unlock the exit door.
As Miguel drives the truck, the trapped prisoners desperately seek an escape route. Rei manages to enter a storage compartment beneath the truck, attempting to open a hidden panel. Meanwhile, Montesinos, having recovered from his previous state, begins an intense chase of the truck after discovering Miguel's jeep, which has photos of his missing daughter. In a frantic pursuit, Montesinos catches up only to be forced off the road, leading to his tragic demise. In the storage compartment, Rei meets a grim fate, impaled on a screw as the vehicle swerves violently.
Facing the dire circumstances, Martín interrogates Nano, who feigns ignorance but unexpectedly mentions someone named Chino, a childhood friend. The tension escalates when Miguel drives the truck onto a precarious frozen lake, shooting the ice and causing the vehicle to start sinking as he exits. In a desperate bid for survival, Martín persuades Ramis, a former band member from his wedding, to assist in his escape through an emergency exit. Together, Martín, Nano, and Ramis make it out, but Golum, another prisoner, succumbs to the bitter cold.
As Nano flees, Martín and Ramis go their separate ways. Martín eventually reaches a nearby village and hears gunfire echoing through the air. He finds himself in the midst of a confrontation between Miguel and Nano. Miguel, fueled by vengeance, reveals the heart-wrenching backstory – his young daughter was allowed to visit a fair with friends, where she tragically fell victim to Nano and Chino, who intoxicated and tortured her to death.
Refusing to permit Miguel to exact his revenge on Nano, Martín intervenes in a heated struggle for a shotgun. Gaining control, Martín aims to defend Nano by shooting him in the foot and hand, demanding the location of Miguel's daughter. Under pressure, Nano divulges that they buried her in a well on a distant farm. Just as the truth unfolds, the sound of a police rescue helicopter fills the air, signaling hope.
In the film's poignant conclusion, Martín takes a moment to gather his belongings from the police station locker, marking a bittersweet farewell as he walks away, likely leaving behind his career as a police officer for good.
