Christine Day, a young opera singer in modern-day Manhattan, searches for a distinctive piece to perform at her next audition. Her friend and manager Meg uncovers an old score, Don Juan Triumphant, penned by a mysterious composer named Erik Destler. Curious, Christine and Meg dig into Destler’s past and uncover whispers of murders and disappearances tied to a young soprano he was rumored to obsess over. When Christine studies the torn parchment on her own, a surge of red-tinged ink seems to bleed from the notes and across her hands, only to vanish as Meg returns. She later auditions with the piece, but a falling sandbag during the performance knocks her unconscious and shatters a mirror, leaving her shaken and silent.
She awakens in London in 1885, dressed in opera garb, with a mirrored, alternate version of Meg by her side. Christine discovers she is the understudy to the diva La Carlotta, a figure both jealous and resentful of Christine’s rising talent. Across the season, the enigmatic Erik Destler — who could be the notorious figure behind the rumor — lurks in the shadows, his presence felt as he strikes at danger from above, even wounding a scene-shifter named Joseph with a blade before blaming him for an accident that nearly harmed Christine. In her dressing room, Christine hears a voice that claims to be her teacher and an angel sent by her deceased father. Erik Destler counsels her to practice Carlotta’s part of Marguerite in Faust, insisting that only she can sing the role.
That evening, Carlotta discovers Joseph’s skinned (yet barely alive) body in her dressing closet, a horrific scene that makes her scream and lose her own voice. Christine is soon cast in Marguerite, a decision that agitates the opera-house owner Martin Barton, who values Carlotta’s prestige and reputation for drawing crowds. In a flashback-like moment, Destler remembers a time when he unknowingly sold his soul to the Devil in exchange for public adoration of his music; the Devil grants the wish but disfigures Destler’s face, telling him that only his music will be loved. Christine delivers a stellar performance that earns a standing ovation, and she celebrates with her fiancé Richard Dutton. She confesses to him about a shadowy “teacher,” though he grows mildly jealous and asks to meet this mysterious mentor, which Christine brushes off as nonsense.
Meanwhile, Destler seduces a prostitute and pays her to impersonate Christine for a night, a chilling reminder that his reach extends beyond the stage. The day after, a harsh review by famed critic E.A. Harrison surfaces in the papers, secretly orchestrated by Barton. Destler tracks down Harrison and brutally murders him in a Turkish spa after the critic refuses to recant. Christine mourns at her father’s grave, praying for guidance as Destler appears as a shadowy violinist and offers her a chance at musical immortality if she will join him. Against her better judgment, Christine accompanies him in his stagecoach, moving deeper into his hidden world.
In the labyrinthine sewers beneath London’s opera house, Destler reveals himself as the composer of Don Juan Triumphant, a trigger for Christine’s memory of the opening lines she sang at the start of the story. He places a ring on her finger and warns never to see another man, a threat that makes Christine tremble but also sparks a stubborn resolve. Richard seeks help from Inspector Hawkins, who reveals a chilling truth: the Phantom is Erik Destler, a man who has lived for decades, using the catacombs as a hideout and skimming his victims to cover his own grotesque face. The only rumored way to destroy him is to destroy his music.
At a masquerade ball, Destler — disguised as Red Death — decapitates Carlotta and abducts Christine, prompting a furious pursuit by Hawkins, Richard, and the rat catcher he has bribed. Back in the Phantom’s lair, Destler tries to rape Christine, but the approaching lawmen force him to retreat. He tells Christine this is “either a wedding march or a funeral mass. You decide which.” Richard and Hawkins burst in, and after a brutal struggle, Richard is wounded and killed, while Christine fights to escape. A desperate bid to end Destler’s reign culminates when Christine uses the lair’s candles to set the place ablaze, and a final shot from Hawkins shoots Destler, while Christine’s escape is sealed by a miracle of fire and smoke.
Christine awakens back in present-day Manhattan and meets the opera’s producer, Mr. Foster, who appears to be Destler in disguise. Foster unveils his true identity and moves to win Christine with a kiss, while Christine discovers a copy of Don Juan Triumphant’s score. In a tense confrontation, she tears away his mask, stabs him, and escapes with the music, letting his life’s work fall into a drain. As she walks the city streets, a street violinist begins to play the Don Juan Triumphant theme. Christine pauses to listen, then steels herself and resumes her journey, wary but determined that Destler’s shadow may still linger, even as she walks on.
