Trama

Paris, 1830: Vidocq is killed by a mirror masked man. A thief turned investigator, he was working on a case of men killed by lightning. His biographer tries to solve the case.

Paris. 1830. In the heart of the town, Vidocq, a famous detective, disappeared as he fights the Alchemist, an assassin that he has been pursuing for a few months. His young biographer, Etienne Boisset, decides to have revenge on Vidocq's death and takes the investigation on...—Anonymous

In 19th century Paris, the famed detective Vidocq (Gérard Depardieu) disappears while pursuing a murderer called the Alchemist.

Etienne Boisset (Guillaume Canet), a young biographer of Vidocq's, follows in his footsteps and progressively uncovers Vidocq's investigations. He learns how Vidocq was first called in to investigate a series of assassinations by lightning, which led him to pursue the Alchemist. Etienne discovers that the Alchemist is a wizard of sorts who wears a mirrored mask and kills virgins to maintain eternal youth. Even the sex of the Alchemist is a mystery, because it sometimes utters mocking feminine sighs during combat with Vidocq. Etienne probes deeper into Vidocq's investigations and eventually comes to learn that the Alchemist uses virgin blood to make magical mirrors that consume human souls.

At last, Etienne comes to the end of his trail and discovers Vidocq alive and in hiding. Etienne dons a mirrored mask and reveals himself to be the Alchemist. Vidocq has known all along, however, and is ready to fight his nemesis. During their final confrontation, Vidocq defeats the Alchemist, but the end of the film suggests that the Alchemist survives.