The film is shot in a found footage style and begins following a reporter named Angela and her cameraman Pablo. They are filming a nightshift with a local Barcelona fire station for a reality TV show. They go out on a call to an apartment building with some firefighters. An elderly woman lives in one of the apartments who is screaming and seems delirious.
As they try to calm her down, she violently attacks one of the firefighters, biting him. The other residents become disturbed by the commotion and chaos ensues. Soon it becomes clear that there is some sort of violent infection rapidly spreading among the building’s residents, causing them to lash out and attack others.
The building is quickly quarantined by health authorities who have arrived on the scene, trapping Angela, Pablo, the firefighters, and the residents inside. They discuss the situation with local police officers who are outside. A health inspector informs them that the building has been sealed off and no one is allowed in or out until the source is determined.
Angela interviews some of the building’s residents and learns that the old woman was brought in by her doctor earlier, who then left in a hurry. As they investigate the woman’s apartment, she leaps out and bites Pablo before being restrained. Pablo’s injury begins mutating rabidly, and he soon becomes violent and attacks the others.
The residents explain that the old woman was the first infected, but everyone she bit subsequently became infected too, turning violent and biting more in a chain reaction. The health inspector aware of the spreading infection orders the police to shoot anyone leaving the building on sight.
A team of firefighters in heavy protective suits enter the building to evacuate the survivors to a medical inspection point set up outside. They begin leading the survivors out through the lower sewer tunnels to bypass the outside police threat. However the infecting contagion follows them and causes more violent outbreaks.
One of the firefighters is killed in an attack while saving Angela. The survivors become trapped in a penthouse apartment as more residents turn into violent savages, their numbers rapidly multiplying. In a scuffle, Pablo is thrown from the balcony of the apartment to his death.
The remaining four survivors struggle to signal their location to the outside as the infected pound on the apartment door. In the apartment, they find documentation about a little girl who was demonically possessed but died there recently after an exorcism attempt. They realize the contagion is supernatural rather than medical in nature.
Barricading themselves in the apartment, the final three survivors – Angela, Manu, and a firefighter named Alex, discover the emaciated corpse of the possessed girl locked away in the room. Manu is lured away when he hears his girlfriend’s voice and is killed by the infected.
As the possessed force their way into the apartment, Alex helps Angela climb into an attic crawlspace to hide while he unsuccessfully tries fending them off. Trapped in the dark attic, Angela is attacked by the possessed girl as the camera footage cuts out, leaving their fate unknown. The film ends with emergency TV broadcasts reporting that the entire apartment building has been quarantined after a massive outbreak.