Sara is a witch who works as a housekeeper for her goddaughter Elena and Elena's husband, a doctor named Eduardo. Sara and Elena learn from a magic mirror that someone seeks to murder Elena, and the mirror reveals that the would-be killer is Eduardo. The mirror also shows the face of a woman whose identity neither Sara nor Elena recognize, but Sara treats her as Elena's "rival."
As Elena prepares for bed, Eduardo poisons a drink and brings it to her bedroom. [Sara] implores the spirits to shield Elena from harm, but a figure intones that Elena must die and that Sara must not interfere. Elena drinks and collapses, her body laid to rest in a cemetery. Through the mirror, Sara speaks with Elena's spirit and vows to avenge her death.
Sometime later, Eduardo remarries and introduces Deborah to his new wife. He shows Deborah around the house, where Deborah feels an icy draft in Elena's former room despite the window being shut. The next day, Deborah plays a tune Elena used to love on the piano, upsetting Eduardo. When Deborah attempts to replace tuberoses—Elena's favorite flower—with daisies in Elena's room, the daisies wilt at once. Deborah asks Sara to help remodel Elena's room and burn Elena's possessions.
Deborah discovers a diary Elena kept without Eduardo's knowledge, on a bookshelf. She tries to show it to Eduardo, but it vanishes before she can. Strange occurrences escalate: the fireplace dies, and the piano seems to play Deborah's favorite tune by itself. Increasingly frightened, Eduardo and Deborah conclude that Elena's vengeful spirit is haunting them. Deborah enters Elena's room and sees Elena in the mirror, causing her to faint. Eduardo follows and, upon seeing Elena in the mirror, hurls a lantern that shatters the mirror and starts a fire that badly scar Deborah's face and body.
Deborah's head and hands are wrapped in bandages. With help from an assistant, Gustavo, Eduardo attempts to reconstruct Deborah's face through skin grafting, using skin from three cadavers—two from a morgue and one from a funeral parlor. When a local pianist dies suddenly, Eduardo plans to replace Deborah's burned hands with those of the pianist. He and Gustavo dig up the pianist's coffin, only to find she was buried alive. In his lab, Eduardo sedates the pianist and amputates her hands. Elsewhere, Sara urges Gustavo to surrender to the police and expose Eduardo's crimes.
While Eduardo is away, Sara casts the severed pianist hands into a furnace, and Elena's disembodied hands take their place. Eduardo then transplants the pianist's hands onto Deborah, who violently tries to strangle him and plays Elena's tune on the piano. After encountering Sara reciting to spirits, Deborah rushes into Eduardo's lab and discovers the corpses Eduardo used for his procedures. Disgusted, Deborah denounces Eduardo as a murderer, until his removal of the bandages reveals that he has perfectly rebuilt her face.
Police, alerted by Gustavo, break into the house. As Eduardo embraces Deborah, her transplanted hands become possessed and fatally stab Eduardo in the back with scissors. The lawmen and Gustavo burst in as Deborah's face and hands revert to their disfigured state, and her hands seize the scissors to kill Gustavo before vanishing. In a final room, with vengeance fulfilled, Sara vanishes into the air.
