During the
French Revolution, Marquis d'Apcher writes his memoirs in his castle. He recounts his experiences in 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, 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 to 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 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 the 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 d'Apcher 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. ==Cast==