Reporter Ángela Vidal and her cameraman Pablo are covering the night shift in one of Barcelona's local fire stations for the television series While You're Sleeping. The firehouse receives a call about an old woman, Mrs. Izquierdo, who is trapped in her apartment. Ángela and Pablo accompany two of the firefighters, Álex and Manu, to the apartment building on Rambla de Catalunya. The old woman becomes aggressive and attacks one of the police officers, biting his neck. The crew find that the police and military have sealed off the building and trapped them inside.




As the residents begin to panic, Álex, who remained upstairs with the old woman, is thrown over the staircase and critically injured. Mrs. Izquierdo kills a girl, and the remaining officer, Sergio, is forced to shoot her. Ángela and Pablo begin interviewing the residents. One of them is a sick little girl named Jennifer. Her mother Mari Carmen claims she has tonsillitis, and says her dog, Max, is at the vet. A health inspector in a hazmat suit arrives to help but the injured become extremely aggressive. The health inspector explains that they are infected with a virus similar to rabies; the disease was traced back to a dog in the building, and Ángela realizes it must be Max. Jennifer, now infected, bites her mother's face and flees. Sergio handcuffs Mari to the stairs. They find Jennifer but she bites Sergio. The infected break down the warehouse door that they were locked in and more of the residents are bitten and infected.




Ángela, Pablo, and Manu, the remaining three survivors, set out to find a key that will allow them to escape the building via a large drain in the basement. Manu is bitten, forcing Ángela and Pablo to take refuge in the penthouse. They discover a tape recorder that explains that the penthouse owner, an agent of the Vatican, was charged with the task of isolating an enzyme carried by a young Portuguese girl named Tristana Medeiros, whose symptoms suggest a demonic possession. The enzyme mutated and became contagious. The agent sealed Tristana in the house to die.




An infected boy damages Pablo's camera and he activates the night vision. A now-ghoulish Tristana emerges and searches the penthouse for food. Pablo is killed by Tristana. Seeing Tristana eating him, Ángela panics and drops the camera. She is then dragged into the darkness screaming.