The film begins with a written disclaimer:
"During the film you are about to see, you will be subject to subliminal messages and mild hypnosis."
"This will cause you no physical harm or lasting effect, but if for any reason you lose control or feel that your mind is leaving your body -- leave the auditorium immediately."
The disclaimer is accompanied by a narrator, who advises viewers to take caution regarding their surroundings once the film has begun, and not to engage in conversation with any unknown individuals for the duration of the running time.
In the opening scene, John (Michael Lerner) is at home with his mother. While he is playing with the pet pigeons in their cages, a bird flies out and around the room, until he manages to catch it and return it to his mother. John goes to work at an eye clinic where a doctor (Nat Baker) shows John and a group of students (Gustavo Gili, Antonio Regueiro and Joaquín Ribas) a collection of eyeballs in jars. John then goes to a patient, named Caroline (Isabel García Lorca), whom he tries to help put contact lenses in her eyes. John leaves after Caroline makes a comment about his eyes and a nurse (Patrice Manget) tells her that John is sensitive to such comments because John is a diabetic and he is slowly losing his vision. Caroline continues her complaints, and from home John's mother (Zelda Rubinstein) seems to listen to the conversation through a shell. "You'll be sorry!" John's mother says.
Later, John is at home eating dinner and his mother plays a spiral on the turntable, takes off John's thick-lenses glasses, and relaxes him through hypnosis. "Now you are with Mommy as one." John visits Caroline at her house, telling her that he has the correct prescription for her eyes. John lets himself into the house to ask Caroline to let him try out her new contact lenses. After fitting them in, Caroline seems content, so John apologizes about that morning and he suddenly pulls out a scalpel and slashes Caroline's throat. When Caroline's boyfriend (José María Chucarro) walks into the room and sees Caroline's dead body, John throws the scalpel at him and then stabs him in the stomach. At home, John's mother gets a phone call from the hospital. She says that John is not coming back (a conversation that John "hears" through his merging of minds with his mother. John's mother thinks that the entire hospital is against John and she tells him via telepathy to cut out all people's eyes.)
As John is cutting out the dead Caroline and her boyfriend's eyes, the image slowly pulls back to reveal that this whole thing is a movie being shown to a small audience in a small movie theatre. The slasher film is called The Mommy and it is having a matinee showing in this theater somewhere in Los Angeles. Among the audience are two teenage girls named Patty (Talia Paul) and Linda (Clara Pastor). Patty is disgusted and frightened by the film and of John's character. The unfeeling Linda, munching on popcorn, tells Patty that it is only a movie.
In the film, John is called home by his mother and she confronts him to kill all of the people in the city saying "all of the eyes of the city will be ours".
Patty begins to complain about her eyes becoming irritated from her own contact lenses, and she sees a weird looking man (Ángel Jové), sitting across the aisle from her also rubbing his eyes.
In the film, Mother begins an eerie trance for John involving spirals, and many people in the theater audience seem dazed by the movie. Patty tells Linda that she feels dizzy, and her breathing gets heavier, until finally the on-screen trance is over and John stands up to leave and tells his mother, "they are going to be very sorry, Mother." John goes out and goes to a movie theatre that is playing The Lost World (1925), sits behind a man, stabs him with a screwdriver and cut out his eyes with a scalpel. Patty finally leaves Linda to watch the film alone.
Mother tells John to go for more people in the theater. John sits behind a couple as the woman (Diane Pinkley) goes for more popcorn, and he kills the unaccompanied husband (Benito Porcino).
In a women's room stall, Patty sees a pair of shoes outside the door and when they are gone, she runs back into the theater and tells her that the weird looking man was in the bathroom. Linda points him out, still sitting in his chair. But Patty is convinced that a man was in the bathroom and she persuades Linda to go check. Linda, who wants more popcorn anyway, finally agrees to check it out and tells Patty to stay to tell her what happened.
In the film, the woman returns to her seat and offers popcorn to her dead husband (which John accepts). John then puts a cloth around her face.
Patty continues staring at the weird looking man, who looks at his wristwatch as if timing the movements in the movie. Patty becomes more uneasy when he stands up and leaves the theater.
In the movie, Mother tells John to come home, but John refuses saying that he can do all this alone. He goes to a restroom and begins to count his eyeballs.
In the theater lobby, the girl at the concession stand (Josephine Borchaca) and the woman at the ticket booth (Antonella Murgía) are shot and killed by the weird looking man who has a silenced .38 caliber semi-automatic pistol. Linda comes out of the ladies room to see the killer dragging the bodies away. A terrified Linda takes cover in the men's room and hides in a stall while the killer enters to hide the two bodies in a stall right next to hers.
In the movie, a woman confronts John for being in the ladies room and he chokes her to death, and in the struggle loses his glasses when they fall off and break on the floor. Mother continues to plead with John to come back home, but John refuses. The ticket lady enters the bathroom and screams when she sees John hanging over the stall and is swiped with the blade. John goes to the lobby for some popcorn and then attacks the concession girl as well.
In the real world, the killer reloads his gun and he seems to be mimicking John's actions on the screen. A man sitting behind a very anxious Patty asks her if she is okay, and she tells him that her friend has been gone for a long time. The man asks Patty what her friend's name is and says that he will go to find her. The man enters the now deserted theater lobby and enters the men's room where he is shot through the neck by the killer. In parallel to both movies, the killer and John both secure the theater doors as the killer in the real world says "Don't worry Mother. I won't leave" and as John in the movie does so, he is caught by the projectionist who has entered the lobby for his break. John chases him and tries to kill him but is unsuccessful. The projectionist is soon seen calling the police. In the real world, a terror-stricken Linda slips out of the men's room and manages to get outside. Linda stops a passing salesman in the street and tells him that there is a murderer in the theater. The man walks with Linda to the theater where he goes inside, sees the secured doors, and blood on the floor. The man runs back out and the two of them run down the street to call the police.
Back in the theater, the woman who was with the helpful man tells Patty that she will be right back for her husband has not returned. The woman is grabbed by the killer when she walks past a curtain and he shoots her as well. The killer then grabs Patty and holds her hostage while she is still in her seat, further mimicking John's onscreen dialogue.
In the movie, a woman in the audience finds the results of John's killings and panic ensues. John stabs a man who tries to get out the secured fire escape door and also failed to reason with John. He quickly grabs the man's teenage daughter hostage and stands in front of the theater screen which continues to screen The Lost World. John screams out for his mother to come get him and Mommy leaves her house and takes a taxi to the theater. She tells the driver to call the police to save the people John has in the theater.
At the same time, the killer then yells at the on-screen John when a man in the audience stands to confront him and is shot dead. Panic breaks out as the killer begins shooting at other people and he drags Patty to the front of the screen. Outside in both movies, the police arrive and surround both theaters. In the old movie, the police break into the theater and Mommy enters with them and tries to reason with John still holding the teen girl hostage in front of the theater screen.
In the real world, SWAT teams enter the theater through the roof and cut the movie's sound and tell the killer to let Patty go. The killer refuses saying that he has to wait until his mother comes to get him. He shoots at the screen at John, and thus drops Patty to the floor. The police sharpshooters then open fire, killing the man. When Patty looks at the screen to see Mommy hypnotizing John out of his trance, a policeman opens fire, and accidentally hits Mommy instead and John cradles her. Suddenly, John looks right at Patty and address her directly saying "I want your eyes too!" He throws the scalpel out of the screen and hits Patty right in her left eye. The police break into the theater and all of the terrified patrons flee. They find Patty on the ground screaming hysterically (but with no scalpel in her eye). They get her outside where she is reunited with Linda and they take her to a hospital for evaluation. In the old movie, John is taken into police custody, and will presumably be placed in a mental hospital. The last shot of the old movie shows the chief of police looking upon Mommy's corpse.
At the hospital, in the real world, Linda and Patty's parents are with her relieved that Patty is fine and just a little emotionally shaken from her horrific experience. When Patty's parents leave, Linda stays a while longer. A little later, Linda tells Patty that she will return tomorrow. In a nearby elevator, Linda is suddenly grabbed by an unseen orderly and a scalpel is aimed at her throat. The orderly walks into Patty's room with a tray and she sits up in her bed and tries to scream. The orderly is John.... and he wants her eyes! John tells Patty as he begins to examine her eyes, "like the doctor said, it's all in your imagination. I really don't exist." Patty finally manages to scream.
During the closing credits, it's revealed that Patty's story was also a movie within a movie. The theatre audience watching this movie leaves the theatre, and the last patron (Jaume Figueras) leaves for the left.