During the French Revolution, Marquis d'Apcher writes his memoirs in his castle. He recounts to 1764, when a mysterious beast terrorized the county, or historical area, of Gévaudan. Grégoire de Fronsac, a knight and the royal naturalist of King Louis XV of France, and his Iroquois companion Mani, arrive to capture the beast. Fronsac becomes interested in Marianne de Morangias, the daughter of a local count, whose brother, Jean-François, was also an avid hunter and a world traveller, whose arm was mangled and rendered useless while overseas. Fronsac is also intrigued by Sylvia, an Italian courtesan at the local brothel
While investigating another victim, Fronsac finds a fang made of steel. A traumatized child witness swears that the beast is controlled by what seems to be a human master. As the investigation proves unfruitful, the king's weapons master, Lord de Beauterne, arrives to put an end to the beast, and Fronsac is sent back in Paris. He realizes that the beast is actually an instrument of a secret society: The Brotherhood of the Wolf, which is working to undermine public confidence in the king and ultimately take over the country. Back in Gévaudan, the attacks by the real beast continue, and Fronsac returns to put an end to the beast's killings. At a secret rendezvous with Marianne, they are attacked by the beast, where it mysteriously refrains from attacking her.
Fronsac, Mani, and a young Marquis set out into the forest and set up an array of traps to capture the beast; it is severely injured but escapes. Mani sets off alone in pursuit, where he finds a catacomb used as the beast's holding pen, inhabited by the Brotherhood. Outnumbered, Mani is shot and killed. Fronsac discovers Mani's body and performs an autopsy, finding a silver bullet—Jean-François' signature choice of ammunition. In a fit of rage, a vengeful Fronsac goes to the catacombs and slaughters many members, but is overpowered by the local authorities and imprisoned.
Sylvia visits him in jail and reveals that she is a spy for the Holy See. She explains that Henri Sardis, the local priest and leader of the Brotherhood, believes that he is restoring worship of God to France. Pope Clement XIII has decided that Sardis is insane, and has sent her to eliminate him. She then poisons Fronsac, saying that he knows too much. Meanwhile, Jean-François comes to Marianne's room and reveals to her that he is the beast's master; it recognized his scent on her when it came near her, which is why it did not attack. He then rapes her when she rejects his advances.
Sylvia's agents exhume Fronsac, who had not been killed but merely put into a temporary coma, and he appears at one of the Brotherhood's sermons. He kills several members, including Jean-François, who reveals that he had regained use of his supposedly mangled arm. Sardis escapes into the mountains, but is mauled to death by a pack of wolves. Fronsac and Marquis go to the beast's lair, where it lies severely wounded. It turns out that the beast was a lion that Jean-François brought back from Africa as a cub that was tortured into becoming vicious and trained to wear spiked metal armor. Fronsac takes pity and kills the beast in an act of mercy.
Marquis finishes writing his account just before he is led to his execution by a revolutionary mob. He states that he doesn't know what happened to Fronsac and Marianne after the death of the beast; but he hopes that somewhere, they are happy together. A final scene shows Fronsac and Marianne sailing on a ship named Frère Loup—Brother Wolf.
Samuel Le Bihan as Knight Grégoire de Fronsac
Vincent Cassel as Jean-François de Morangias
Émilie Dequenne as Marianne de Morangias
Monica Bellucci as Sylvia
Jérémie Renier as Marquis Thomas d'Apcher
Jacques Perrin as older Thomas d'Apcher / Narrator
Mark Dacascos as Mani
Jean Yanne as Count de Morangias
Jean-François Stévenin as Father Henri Sardis
Édith Scob as Countess Geneviève de Morangias
Johan Leysen as Antoine de Beauterne
Bernard Farcy as Intendant Pièrre-Jean Laffont
Hans Meyer as Marquis d'Apcher
Philippe Nahon as Jean Chastel
Gaspard Ulliel as Louis
Nicolas Vaude as Maxime des Forêts
Virginie Darmon as La Bavarde
Eric Prat as Captain Duhamel
Jean-Loup Wolff as Duke Gontrand de Moncan
Dee Bradley Baker as Beast vocal effects