Miguel, a young man with a disfigured face, schemes to lure a girl into sex by donning a mask and posing as her boyfriend. When she unmasks him, he uses a pair of scissors to brutally stab the young woman. After the crimes, Miguel is sent to a mental asylum for five years. When his sentence ends, he is released into the care of his sister Manuela. Along with their invalid aunt and the Countess Maria Gonzales, his sister Manuela runs a boarding school for young women, Europe’s International Youth-Club Boarding School of Languages, perched on the sun-soaked Costa Del Sol.
On the train home from the sanitarium, Miguel notices a long-haired brunette named Angela. At the same time, Alvaro arrives at the school to visit [Manuela], while the countess warns that she is being plotted against and that Manuela only seeks money. That very night, the countess is burned to death in her bed. Angela arrives at the school later than her friends and ends up staying in the room where Miguel killed the girl. While she cleans the bathroom mirror, she glimpses Miguel’s reflection and bolts, with the killer in hot pursuit. She is taken in by her friend Inga, and the two visit Laura, Eva, and the gardener Antonio. Miguel goes to see his sister and openly declares his love for [Manuela], resulting in a kiss and touch. Manuela pulls back, recalling the incident from years past and insisting that nobody understands them.
After a night at the disco, [Antonio] escorts Angela home. While Angela reads in bed, the killer slips inside the room; [Eva] appears at her window to ask for a pullover. As Eva changes, the killer strikes, stabbing her in the back and then ripping the knife out of her chest before she dies. The next day, Alvaro’s Spanish class listens to dialect tapes. On Angela’s tape, the voice suddenly changes and menaces her with a hacksaw. She screams when she sees Miguel at the window, but he vanishes when Alvaro returns, and the tape resumes its lessons. After discovering bloody clothing at Eva’s place, Angela sees Antonio arguing with Manuela before a boulder tumbles down a hill and nearly crushes Angela. Manuela notices one of her snakes is missing, and Angela searches for Eva before fleeing after spotting Antonio killing a snake with garden shears. Alvaro appears and tells her to stop bothering his students.
Laura and her friends go to Angela’s room, where Inga pretends an intimate act. They don’t notice Eva’s corpse hanging in the closet, but the girls see Inga through the window and mock her. Outside, Angela sees Inga drive off in a yellow car similar to Manuela’s. Inga accompanies the masked killer to a remote location, where she is tied with rope. The killer starts a saw and pushes Inga toward it; after a boy momentarily turns the machine off, the saw comes back to life and Inga is decapitated, while the killer chases the boy and runs him down.
Meanwhile, Angela packs her things and spots Antonio at the front door and Miguel at the window; she panics, barricades herself, and lets Laura in, who offers to stay for the night. Laura slips away to the disco for a drink and returns to find the killer crushing her neck with an iron collar. After a threatening phone call, Angela discovers the bodies of her friends and is attacked by the killer. While Miguel lies unconscious during the struggle, Angela sees Alvaro driving away in his car. She runs to [Manuela] to report what happened, and Manuela leads her to a room to calm her with a drink.
Manuela and Alvaro discuss the killings, and Miguel overhears them plotting to pin the murders on him to preserve the family inheritance. Manuela also notes how Miguel believes she loves him and his disfigurement. She tells Alvaro that she has already been paid and orders him to kill Angela. Miguel finds a room and, believing Angela is his sister, chokes her until she shoves a spike into his neck. Angela escapes to the stairs and sees Alvaro and Manuela in a struggle, before screaming at the body of the countess found in another room. Alvaro bursts into the room with a knife, but Manuela intervenes and kills Alvaro with a hedge cutter, slapping Angela’s throat with the blade and reminding her that she saved her life; Angela nods and runs away. Manuela returns to her room and speaks to her deceased brother, but Miguel eventually sits up and strangulates Manuela. The film closes with Miguel collapsing, clinging to his sister’s hand as [Antonio] embraces Angela and the police arrive.
