Plot

A Paris model must return to Madrid where her grandmother, who had brought her up, just had a stroke. But spending just a few days with this relative turns into an unexpected nightmare.

The movie opens with an elderly woman entering an apartment, where she finds another older woman lying dead on the floor, and a half-eaten birthday cake on a table. She is then greeted by a young, blond naked woman. The old woman seems happy to see her and kneels down on the floor and kisses her. The film then focuses on Susana, a fashion model working in Paris; she calls her grandmother Pilar and tells her she will not be in Madrid for her birthday. A short time later, while snorting a stripe on her cellphone, Susana receives a call from a hospital in Madrid, telling her her grandmother had a brain hemorrhage. Being Pilar's only remaining living relative, Susana travels to Madrid; it is then revealed that Pilar is the same elderly woman from the opening sequence.

Back in Madrid, Susana takes the seemingly brain-damaged Pilar back to her apartment in Calle de Alfonso XII (next to El Retiro) and moves in to take care of her. As the days go by, Susana begins having several disturbing nightmares, making her uneasy. One day, Susana finds a young woman named Eva visiting Pilar. Eva tells her she is the niece of Pilar's best friend Julita, and that she and Susana have met before, but Susana does not remember her nor Julita. One night, Susana comes across her childhood diary; after reading it, she recalls a birthday party where she met both Eva and Julita. On that party, the two girls underwent some sort of strange ritual and both got locks of their hair cut; Susana also remembers that, from that day on, she began having nightmares related to her grandmother. She also begins to suspect that Pilar is moving through the apartment at night.

Growing increasingly disturbed, Susana tries to leave and find accommodation for her grandmother, and in doing so, realizes that Pilar is a witch with supernatural powers. She causes the death of Adela, a domestic worker Susana had hired to take care of her. Then, the same day Susana takes her to a nursing home a fire breaks out and kills all the guests in the house, whilst Pilar is seen back at her apartment without further explanation. Determined to go back to Paris, Susana kills Pilar by poisoning her soup. Minutes after her grandmother's death, the phone rings: it is Eva, who hears Susana's youthful voice and calls her Pilar. Susana responds by saying that she is not Pilar, to which Eva replies: "Are you sure?".

Shortly after, Pilar uses her powers to move things around the house and keep Susana from leaving. Susana locks herself in the bathroom where she finds a painting of Pilar in her youth; she is then attacked by Pilar, who uses her powers to immobilize her to the floor. Pilar opens her mouth and transfers her soul to Susana's body. Susana, now possessed by Pilar, goes to look at the painting, which now shows Susana. Shortly after, Eva -revealed to be the young woman from the beginning- appears. The two women caress each other intimately and the film ends.