1906. In a crypt-like room, the hunchbacked Franz von Holstein, Luciano Rossi, mourns Greta, his young sister and only love. A first flashback reveals a dark moment between them, where Franz commits an act of sexual violence against Greta, and she then confesses a desperate wish to leave this cursed place, to live among people. In a second flashback, Greta playfully provokes him to chase her when she stumbles upon Doctor Herbert von Ravensbrück, and the two become romantically involved while Franz watches from the shadows, a haunting prelude to the tangled fates that will follow.
1909. Walter von Ravensbrück, Sergio Doria, and his wife Eva, Angela Bo von Ravensbrück, are served tea by their loyal butler Simeon, Marco Mariani, when a carriage hurtles by and overturns, its coachman fatally impaled. Inside the overturned coach lies Greta, unconscious. Inspector Dannick, Attilio Dottesio, would like to question Greta, but Walter persuades him to hold off. Doctor Sturges, Klaus Kinski, is summoned to examine her. He discovers that Greta bears a gold medallion etched with the inscription “Greta 1906” and cryptic symbols that unsettle him.
Gertrude, the Ravensbrück’s maid, Carla Mancini, has already recognized Greta, and Franz von Holstein’s apparition begins to haunt her room, vanishing and reappearing as a menacing presence. He also slits Gertrude’s neck, a wound that bleeds yet mysteriously vanishes, a sign of something beyond the living. Dr. Sturges finds a matching scar on Greta’s neck and tests her with a needle in her right eyeball, which causes no pain and confirms he is dealing with something undead. When he checks the coachman’s death certificate, he opens the coffin to reveal rapid decomposition, another sign that Greta was undead at the moment of the crash. Sturges keeps these astonishing truths secret.
Gertrud packs her belongings and attempts to leave, but is pursued by Franz’s spectral figure and ultimately killed by an unseen assailant. Meanwhile, in his underground laboratory, Sturges works to unlock the medallion’s cryptic symbols—the key to a formula for reviving life. He succeeds in briefly animating one of his corpses, only to be strangled by an unseen force along with his deaf lab assistant and the revived body.
Walter and Eva deepen their relationship as they watch Greta: Eva’s jealousy swells while Greta and Walter share quiet moments. Observing a clandestine meeting between Walter and Greta, Eva’s jealousy erupts. In Walter’s absence, Eva lures Greta into the villa’s cellar, confesses her hatred, and walls her up alive. Eva then lies to Walter, claiming Greta has departed unexpectedly. Inspector Dannick resumes the search, but Greta’s whereabouts remain a mystery.
A month later, a masquerade ball fills the villa with shadow and scent of danger. During a party game, Eva is asked to guess the identities of the masked guests. A woman in red, whose name Eva cannot guess, discreetly reveals herself when her mask comes off: Greta, who vanishes as she unburdens herself of her disguise. Eva checks Greta’s chamber, only to be confronted by a dark cat that leaps through a wall opening and leads her to Greta, whose face morphs into a rotting corpse. Greta stalks Eva to the attic, where Eva dies screaming.
Herbert von Ravensbrück, Walter’s father, returns for Eva’s funeral and glimpses Greta at a distance, triggering a flood of memories from 1906: Greta dies in childbirth, and Franz points a finger of blame at him. After Eva’s funeral, Herbert lingers by Greta’s grave. Greta emerges from the shadows and taunts him about the unborn child’s death, asking for a kiss; her face again becomes rotten, and he flees into a crypt. The door slams shut, and Eva’s newly risen corpse advances toward him.
Back in Walter’s room, a dark cat slips in, and Greta sits at his bedside, creeping into his bed as her visage turns to putrid flesh. In the granary, Greta lures Simeon out of hiding and promises him money, or even love, if he keeps her identity secret, then kills him. The police cover up the body, while Walter’s corpse is found clutching Greta’s medallion.
Inspector Dannick seeks answers from Professor Kempte, Fernando Cerulli, who explains that the medallion’s symbols point to a Maya/Incan belief in a life-restoring formula. The brilliant former student Franz von Holstein had pursued deciphering it but abandoned the project after Greta’s death. Kempte then gives Dannick Franz’s address.
In a final, sunlit flashback to 1906, Franz approaches Greta, dressed in white and carrying white flowers. He claims to have revived her and vows they will leave together, placing the medallion around her neck as proof of their “new creation.” Greta throws the white flowers into his face, and they transform into the dark cat, which kills him. Greta, her dress turning red, giggles and leaves with a sly smile. Dannick, entering in 1909, discovers Franz’s corpse.
At the cemetery, Dannick gazes at Greta’s photograph on her empty grave, wondering if he will ever solve the mystery of a woman he never truly saw. Returning home, he finds the figure he believed to be his wife waiting with her back to him turn around: an aged Greta, smiling, and perhaps recognizing her own face in his gaze.
