In eighteenth-century France, aristocratic composer Octave Barron writes a symphony for the Devil as part of a satanic pact. During his work he is “inspired” by the murders he commits with his lover and maid, Carlotta Beverly Garland. For his crimes and devil worshipping, Barron is eventually discovered by the authorities and taken from his mansion by an angry mob of townspeople, who tear him apart with horses while Barron screams that his pact with the evil forces will resurrect him through his music.
Several years later, Barron’s niece Gabrielle Apollinaire Jennifer Burns inherits the estate and discovers his hidden and unfinished symphony, as well as his piano. Believing that all the stories about Barron are nothing but superstition, she hires the church choir director and novice composer Marius Carnot Ben Cross to finish the work. Meanwhile, the now-old Carlotta, who has avoided suspicion and is a secret practitioner of witchcraft, is contacted by her master's spirit from Hell, who directs Carlotta on how to bring him back in the body of Marius. Marius is possessed by Barron’s evil spirit after he plays the latter’s music on the piano, and murders two women for “inspiration” to finish the symphony, while Carlotta secretly helps him to escape and create alibis.
Things grow tense when Gabrielle’s fiancé Julien Doug Wert arrives, clashing with Marius and showing jealousy. Carlotta poisons Julien to provoke a furious reaction that leads him to try to rape Gabrielle and then to challenge Marius to a duel. During the duel, Marius again becomes possessed by Barron’s spirit after hearing the symphony being played by Carlotta, and in that state he defeats and kills Julien.
Fearing the worst, Marius tries to leave Gabrielle’s mansion, but before he does so Carlotta commits suicide, as instructed by Barron. Later, as Marius finishes the symphony, he’s again possessed by Barron, while Gabrielle is possessed by Carlotta’s spirit. They have sex while their reflections reveal they’re possessed. Carlotta’s spirit leaves Gabrielle’s body and she goes to Hell as punishment for her suicide and crimes, as Barron intended from the beginning. Barron plans to live in full control of Marius’ body, but Gabrielle confronts her uncle while the police and townspeople storm the place, seeking retribution for the third murder committed by Marius/Barron. A fight between the possessed Marius and Gabrielle ensues, and Marius regains consciousness just before the police and townspeople arrive. Knowing there is no escape from the mental and physical hold Barron has over him, Marius declares his love to Gabrielle and throws himself into the fireplace, burning alive and tragically dying while saving his mind and soul.
Nevertheless, that very same night, when everyone but Gabrielle has left the mansion, Barron resurrects his dead body and comes out of the grave. Gabrielle realizes she and Marius forgot to destroy the symphony that powers Barron. She then manages to survive by burning the symphony and Barron’s piano, while the helpless Barron can only sit and play out a few last moments at the burning piano before finally dying, this time forever. The next day, Gabrielle closes all the doors to the estate and travels alone, briefly looking back before leaving for good.
