Violin virtuoso Rose Fisher and her agent Charles Vernais are thrown into turmoil when they uncover the tragic truth about Rose's estranged father: he took his own life by setting himself ablaze. The revelation is even more shocking as they learn that Rose's father was none other than Richard Marlowe, a once-renowned composer who became an eccentric recluse. Their already strained professional relationship hits a breaking point when Rose chooses to pause her commitments to visit her father's remote French estate, which now belongs to her.
Upon arriving at Marlowe's mansion, she encounters Thérèse, her father's former housekeeper. While in town for cigarettes, Rose feels the weight of her father's notorious legacy as the locals regard her with puzzled glances. Curiosity leads her to discover sheet music for an unfinished violin sonata hidden within her father's desk, adorned with strange occult symbols. When she plays a portion of the music on the piano, a haunting figure temporarily manifests behind her.
In a moment of regret, Rose reaches out to Charles to mend their professional rift. She shares the details of the sonata and sends him images of the cryptic symbols for his insight. Drawn by an eerie presence and a childlike voice, Rose finds herself in the music room again, where she uncovers a clock marked with one of the sonata's symbols. Recognizing the piece as Marlowe's final creation, Charles seeks the expertise of former client and musical prodigy James. James notes that the music is disjointed in parts and, unable to decipher it further, guides Charles to Sir Victor Ferdinand, a music historian who once collaborated with Marlowe.
Investigating the symbols, Victor presents Charles with a book on occult history. He reveals that the strange signs are linked to a clandestine French society that believed music could open a portal to the devil, possibly even summoning the antichrist. However, the deeper they delve, the more Charles suffers from debilitating headaches whenever Victor plays the sonata.
While searching the mansion, Rose finds news clippings of her musical achievements that her father treasured. She also notices one of the occult symbols burned into the fireplace during a fire, leading to a series of nightmarish visions where her father, engulfed in flames, points towards a living statue of cherubs outside. Upon Charles' return to the estate, he informs Rose that the symbols they uncovered each represent one of the four faces of evil: power, immortality, appearance, and duality, collectively forming a fifth, even darker emblem of the occult.
Rose shares her findings with Charles, revealing the symbols found on the clock and in the fireplace. Together, they attempt to reveal hidden musical notes by heating the sheet music, soon realizing the score needs to be played with the clock set an hour ahead. Following Rose's suggestion, Charles seeks out Thérèse again, who recalls seeing another symbol behind a mirror. This leads him to theorize that the sonata should be played backward. Thérèse mentions that Marlowe often vanished into the woods, with the locals suspicious of his involvement in the disappearances of children.
As Rose explores the woods, she stumbles upon an old chapel leading to a chilling underground chamber. There, murals depict demonic motifs, alongside recordings of Marlowe capturing the screams of children. On her return, Rose encounters several ghostly children, which she relays to Charles. As his fixation on solving the sonata intensifies, Rose reveals the duality symbol associated with the cherub statue they saw outside, indicating that the sheet music must be layered over each other for clarity.
With the final symbol—the fifth—unveiled in a rose depicted on a portrait of Marlowe, Charles realizes that the score was written for Rose's innocent soul, intricately composed from the anguished screams of kidnapped children. Horrified by the malevolent nature of the music, Rose attempts to destroy the pages and contact the authorities. However, consumed by obsession, Charles chokes Rose, compelling her to play the sinister sonata.
As she plays, her eyes gleam with an otherworldly light, and ghostly children appear. Tragedy strikes when a demonic entity manifests, claiming Charles's life violently. The final note played shatters the windows, and the following day, Thérèse discovers Charles's lifeless body.
Time passes, and at the prestigious Royal London Concert Hall, Victor witnesses an unsettling performance by Rose. As she begins to play the haunting sonata in front of an audience, her eyes exhibit the glow of possession, hinting that the dark legacy of her father's music may not yet be finished.
