During a formal dinner party at the lavish mansion of Señor Edmundo Nobile and his wife, Lucia, the servants unaccountably leave their posts until only the major-domo is left. After dinner the guests adjourn to the music room, where one of the women, Blanca, plays a piano sonata. Later, when they might normally be expected to return home, the guests unaccountably remove their jackets, loosen their gowns, and settle down for the night on couches, chairs and the floor.
By morning it is apparent that, for some inexplicable reason, they are psychologically, but not physically, trapped in the music room. Unable to leave, the guests consume what little water and food is left from the previous night's party. Days pass, and their plight intensifies; they become quarrelsome, hostile, and hysterical - only Dr. Carlos Conde, applying logic and reason, manages to keep his cool and guide the guests through the ordeal. One of the guests, the elderly Sergio Russell, dies, and his body is placed in a large cupboard. Much later in the film, Béatriz and Eduardo, a young couple about to be married, lock themselves in a closet and commit suicide.
In the meantime, the guests manage to break open a wall enough to break a water pipe. Eventually, several sheep and a bear break loose from their bonds and find their way to the room; the guests take in the sheep and proceed to slaughter and roast them on fires made from floorboards and broken furniture. Dr. Conde reveals to Nobile that one of his patients, Leonora, is dying from cancer and accepts a secret supply of morphine from the host to keep her fit. The supply of drugs is however stolen by Francis and Juana, a brother and sister. Ana, a Jew and a practitioner of Kabbalah, tries to free the guests by performing a mystical ceremony, which fails.
Eventually, Raúl suggests that Nobile is responsible for their predicament and that he must be sacrificed. Only Dr. Conde and the noble Colonel Alvaro oppose the angry mob claiming Nobile's blood. As Nobile offers to take his own life, a young foreign guest, Leticia (nicknamed "La Valkiria") sees that they are all in the same positions as when their plight began. Obeying her instructions, the group starts reconstructing their conversation and movements from the night of the party and discover that they are then free to leave the room. Outside the manor, the guests are greeted by the local police and the servants, who had left the house on the night of the party and who had similarly found themselves unable to enter it.
To give thanks for their salvation, the guests attend a Te Deum at the cathedral. When the service is over, the churchgoers along with the clergy are also trapped. It is not entirely clear though, whether those that were trapped in the house before are now trapped again. They seem to have disappeared. The situation in the church is followed by a riot on the streets and the military step in to brutally clamp down on the rioters. The last scene shows a pack of sheep entering the church in a row, accompanied by the sound of gunshots.