In 1830 Paris, private investigator Vidocq, Gérard Depardieu, pursues a mysterious Alchemist, a hooded figure wearing a mirrored mask. In a brutal confrontation inside a furnace room at a glass factory, the Alchemist lures Vidocq into danger and, during the fight, shoves him toward the blazing furnace. Dangling from the ledge, Vidocq demands to see his foe’s face, and when the mask is finally lifted, he releases his grip and disappears into the fire, leaving only questions and a trail of ash.
Journalist Étienne Boisset begins his own pursuit of Vidocq’s fate, hoping to write a definitive biography and, more urgently, to uncover the truth behind Vidocq’s supposed death. He seeks out Vidocq’s longtime associate and confidant, René Nimier, a steadfast ally who shares the sense that a killer is moving through the city’s shadows. Together they set a broader investigation in motion, drawn by whispers of a case that defies ordinary murder, and by a police chief, Lautrennes, who suspects a deeper plot behind the sudden losses at a famed cannon factory.
The case widens as Tauzet, an ambitious officer, is tasked with watching Boisset’s movements and the lingering mysteries surrounding Vidocq’s supposed demise. Meanwhile, Boisset secretly surreptitiously retrieves the pins connected to the deaths and begins tracing them to the underbelly of Montmartre, where a dancer known as Preah resists easy explanations. Preah, Vidocq’s concealed lover, had received a letter promising fortune in exchange for placing those precise pins in the victims’ hats, and the letter even points toward a third target—the owner of an orphanage, Ernest Lafitte.
Vidocq’s investigations soon illuminate a web of vanity and moral compromise among the city’s wealthy: a trio of magnates—Belmont, Veraldi, and Lafitte—who lusted after a potion promising eternal youth, and who enlisted willing maids for their experiments. The Alchemist, in turn, offered an elixir of youth in exchange for cooperation, but when the trio grew uneasy with their own guilt, the Alchemist killed them to preserve his secrets. The tangled story surfaces through interviews with the brothel’s manager, Sylvia, and through the sorrowful connections of Marine Lafitte, the wife of Lafitte, who becomes entangled in the danger as the bodies pile up.
Boisset’s pursuit of truth draws him into a deadly cat-and-mouse game with the killer, and Tauzet notices a disturbing pattern: the killer is disposing of witnesses and growing bolder with each passing moment. The investigative team discovers Vidocq’s notes, which reveal a hidden lab where a masked Alchemist uses maidens’ blood to fuel a sinister device—one that drains souls to sustain the mask’s power of eternal youth. The killer arrives and attacks Vidocq, and a critical clue—the mask itself—seems to hold the key to the mystery.
In a daring turn, Boisset infiltrates the glass factory with Nimier and Preah, guided by a secret artisan who ultimately reveals himself to be Vidocq. Vidocq had staged his own death, intending to mislead Boisset and draw out the Alchemist’s violent methods. With his cover blown, Boisset dons the Alchemist’s mask, and Nimier falls to a hail of gunfire that is miraculously reflected back toward him by the mask’s mystic powers.
Vidocq relentlessly pursues the Alchemist through a hall of mirrors, where the trapped souls inside the mask begin to emerge. A final confrontation culminates in Vidocq forcing the killer to confront his own reflected horror, freeing the spirits imprisoned within the mask and ensuring the Alchemist is impaled by a shard of mirror and cast into the river. Yet the participants’ uneasy certainty lingers: the body is never found, and the laughter of the Alchemist echoes at Nimier’s funeral, signaling that the danger may not be over and that the mask’s gleaming secret still casts a shadow over Paris.
