During the French Revolution, the Marquis d'Apcher pens his memoirs in his castle, reflecting on a dark chapter from 1764, where a mysterious beast wreaked havoc in the region of Gévaudan. Enter Grégoire de Fronsac, a knight and royal naturalist serving King Louis XV of France, alongside his Iroquois companion Mani. Their mission is to capture this elusive beast. Amidst the intrigue, Fronsac finds himself drawn to Marianne de Morangias, the daughter of a local count, who has a brother, Jean-François, a passionate hunter with a tragic past that left his arm mangled during his travels. Adding to the web of complexity, Fronsac is captivated by Sylvia, an alluring Italian courtesan from the local brothel.
As Fronsac probes the circumstances surrounding the latest victim, he discovers a steel fang—evidence of something more human lurking behind the beast. A traumatized child insists that the creature is under the influence of a human master. Despite the seemingly fruitless investigation, Lord de Beauterne, the king's weapons master, arrives to eliminate the threat, leading to Fronsac's return to Paris. There, he uncovers a sinister plot orchestrated by the Brotherhood of the Wolf, a secret society aimed at destabilizing the king’s power and seizing control of the nation. But in Gévaudan, the true beast’s attacks continue, prompting Fronsac’s determined return to confront this menace.
In a tense encounter at a clandestine meeting with Marianne, the beast strikes, yet curiously spares her life. With Mani and a young Marquis, Fronsac ventures into the woods, setting traps for the creature. Though they severely injure it, it manages to escape. Mani, pursuing the creature alone, discovers a hidden catacomb inhabited by the Brotherhood. Outnumbered, he meets a tragic fate. Fronsac, discovering Mani's lifeless body, performs an autopsy, revealing a silver bullet—the signature of Jean-François. Consumed by rage, Fronsac retaliates against the Brotherhood, but local authorities capture him.
In prison, Sylvia pays a visit, revealing her role as a spy for the Holy See. She discloses that Henri Sardis, a local priest and Brotherhood leader, believes he is restoring God's worship in France. With a command from Pope Clement XIII deeming Sardis mad, she is tasked with eliminating him and subsequently poisons Fronsac, claiming he has too much knowledge. Meanwhile, Jean-François confronts Marianne, unveiling himself as the beast’s master, explaining that it had recognized his scent, explaining its non-aggression towards her. In a shocking turn, he assaults her when she rejects him.
Later, agents working for Sylvia resurrect Fronsac, who then confronts the Brotherhood during one of their sermons, exacting vengeance on several members, including Jean-François—who shockingly reveals his arm is fully functional again. As Sardis attempts to flee to the mountains, he is ultimately consumed by a pack of wolves. Together with the Marquis, Fronsac locates the beast's lair, only to discover that the creature is not merely a monster but rather a lion, tormented and forced to wear spiked armor after being smuggled back from Africa as a cub. In a poignant act of mercy, Fronsac ends the beast’s suffering.
Finally, the Marquis d'Apcher closes his memoirs just before facing execution at the hands of a revolutionary mob. He reflects on Fronsac and Marianne’s fate post-beast, hoping that they find peace together. The narrative concludes with a serene image of Fronsac and Marianne on a vessel named Frère Loup—Brother Wolf.
