Balaguero and Plaza previously directed the 2002 documentary OT: la película. REC was filmed as a found footage film and used a "shaky camera" technique. The film was remade in the US as the 2008 film Quarantine.As the first installment of the REC series, it was followed by three sequels; REC 2 in 2009, REC 3: Genesis in 2012, and REC 4 in 2014 as the final installment in the franchise. Spanish company Filmax International is responsible for the production of the REC franchise and will also release the fourth and final installment.
REC follows a television reporter, Ángela Vidal (Manuela Velasco), and her cameraman, Pablo, who cover the night shift in one of Barcelona's local fire stations for the fictional documentary television series While You're Sleeping. While they are recording, the firehouse receives a call about an old woman who is trapped in her apartment. When they arrive, Ángela and Pablo film the police breaking down the door. As they approach the old woman, she becomes aggressive and attacks one of the police officers, forcing the others to restrain her.
Meanwhile, the building's residents gather in the lobby and look on as the police and military seal off the building. A firefighter who remained in the old woman's apartment suddenly plummets through the stairwell to the lobby floor. The camera crew, remaining firefighter Manu and the second policeman go up again and are attacked, forcing the officer to shoot the old woman. The camera crew remains trapped inside the building with the residents and continue recording. Ángela interviews a little girl named Jennifer who lives in the building. Jennifer is ill with what her mother claims is tonsillitis. She says her dog, Max, is at the veterinarian because he appeared to be sick as well, and her father is out buying medicine.
Soon, a health inspector wearing a hazmat suit arrives and attempts to treat the injured, who become aggressive and are locked inside the building's textile warehouse. The health inspector explains that they are infected with a virus similar to rabies, and the time frame in which the disease takes effect varies by blood type. The health inspector also reveals that sometime during the previous day, a dog with the virus was taken to a local veterinarian; the dog became violent and attacked and killed other pets at the clinic. The dog was euthanized, and was traced back to the apartment building. Ángela suddenly realizes that the dog belongs to Jennifer, the sick child she interviewed earlier. The other residents surround Jennifer and her mother Mari Carmen, and are questioning them about the dog, when Jennifer suddenly vomits blood all over her mother's face, jumps out of her arms and runs upstairs.
The policeman handcuffs Maria Carmen to the stairs and proceeds upstairs with the firefighter Manu. They find Jennifer, and the officer tries to calmly subdue her with a syringe that the inspector gave him, but she bites him and he tells the others to leave him behind as he is now infected. Manu and Pablo run outside where they find that the infected are trying to break into the main hallway through the textile warehouse. They are forced to leave the handcuffed Mari Carmen, since the now-infected officer has the keys, and they run upstairs into an apartment. More and more people in the building become infected, and Ángela, Manu and Pablo are forced to fight them off. Eventually they learn that there is a key to a door in the apartment building workshop, which leads to an exit via the sewer system. However, the key is located on the fifth floor in the manager's apartment.
After finding the key, Ángela and Pablo appear to be the only human survivors, everyone else being dead or infected. Rather than making their way to the workshop, they are forced upstairs to the penthouse by the remaining infected. They search the penthouse and discover that its owner was an agent of the Vatican who was charged with researching and isolating an enzyme believed to be the biological cause of demonic possession, which was later confirmed to exist in a young girl named Tristana Medeiros who was raped by a group of priests. The agent kidnapped and brought the girl to the penthouse to conduct his research and possibly cure her; during this time the enzyme mutated and become viral. The agent, having no other options, decided to seal her off, presumably to let her die of starvation.
A door to the attic opens and Pablo uses his camera to look inside. An infected boy jumps at the camera and breaks its light. Pablo turns on the night vision and discovers the sealed door referred to by the agent on an audio tape. Tristana, now a blind and horribly emaciated figure, emerges and begins searching the penthouse, holding a hammer. Ángela and Pablo try to escape, but Pablo trips and is viciously attacked by Tristana, causing him to drop the camera. Ángela picks it up and runs, only to fall and drop the camera as well. She searches for it in the dark but is unable to find it. The camera continues to record as Ángela is dragged into the darkness, screaming.
- Manuela Velasco - Ángela Vidal
- Ferrán Terraza - Manu
- Jorge-Yaman Serrano - Young policeman
- Pablo Rosso - Pablo
- David Vert - Álex
- Vicente Gil - Adult policemen
- Martha Carbonell - Mrs. Izquierdo
- Carlos Vicente - Guillem Marimón
- María Teresa Ortega - Grandmother
- Manuel Bronchud - Grandfather
- Akemi Goto - Japanese woman
- Chen Min Kao - Chinese man
- María Lanau - Mari Carmen, Jennifer's mother
- Claudia Silva - Jennifer
- Carlos Lasarte - César
- Javier Botet - Tristana Medeiros
- Ben Temple - Doctor
- Ana Velasquez - Colombian girl
- Daniel Trinh - Chinese children
- Marita Borrego - Operadoras Cuartel Bomberos
- Jana Prats - Operadoras Cuartel Bomberos (as Ana Prats)
- Víctor Massagué - Child in attic
- Javier Coromina - Voice of Pablo