At midnight on Christmas Eve in the mid-19th century, somewhere in Russia, two fugitives fleeing persecution stop by the roadside for the woman to have her baby. The mother dies, and the father is slaughtered by wolves. However, the wolves protect the baby instead of killing it, and the baby grows into a wild boy.
Years later, a trio of circus performers find the boy out in the woods, and use him as an attraction called the "Wolf Boy". He is named Etoile, and loses his wolfish aspects, and his public appeal, as he grows up. One night, Etoile changes into a wolfman under the influence of the full moon, and kills a circus member, Tiny. As he is dying, he accuses Etoile, who flees.
He soon arrives in Paris, and becomes assistant to a zookeeper. That same day, a group of prostitutes from a nearby brothel visit to have lunch, and Etoile is smitten by the pretty Christine. She takes a liking to him, but keeps her job a secret. Later, Etoile decides to take Christine dancing, but is turned away by Madame Tellier. He tries to sneak in by the window, but catches Christine in the middle of entertaining a client. He bursts through the window in a jealous rage, and attacks the client. Madame Tellier stops him, and chases him away. Christine confronts Etoile the next morning, and in the ensuing argument, she tells him about her history as an orphan until Madame Tellier took her in. Etoile asks Christine to marry him, but she tells him it would not work. That night, Etoile changes again, and kills clients leaving the brothel.
The attacks draw the interest of Professor Paul Cataflanque, a skilled forensic pathologist, who initially deduces that it was a wolf. He embarks on his own investigation against the protests of his friend, Inspector Gerard, and inspects the wolves in Etoile's zoo. Etoile's demonstration of their gentleness leaves Paul sceptical, as does the new evidence gathered. The evidence leads him to the brothel, and he questions Madame Tellier, who is put out by his requests to identify the bodies. He brings photographs of the victims, and she lies about having seen them. However, Christine sees them also, and Paul, noting her reaction, questions her in private. She admits to having had them as clients, but leaves Etoile out of her story.
Meanwhile, the Prefect of Police decides to make Paul's wolf theory official, and orders all zoos to kill their wolves. Etoile is given the grisly task, and he is beside himself with grief. Christine visits him, and leaves to get the zookeeper, thinking Etoile is sick. Etoile changes, and escapes into the sewer before she returns. With his rage and grief spurring his viciousness, Etoile goes on a killing spree, and hides in the sewers the next day. Paul discovers one of the victims is still alive, and revives her long enough to hear her speak of a creature neither a man nor a wolf. Paul's servant Boulon tells him of the werewolf tales from his countryside home, and Paul deduces the attacker will kill the next night. He interviews Christine again, and asks her to wait in Etoile's room. He gets a map of the sewers, and forges a silver bullet as a precaution.
That night, he goes down into the sewers, and encounters Etoile. Paul tries to reason with him, offering his help. Etoile is temporarily brought to sanity, but Gerard, warned by Boulon, attacks at the last minute. Etoile flees to the zoo, followed by Paul. Christine is shocked and frightened by Etoile's wolf form, but Etoile does not hurt her. Paul tries once more, but Gerard shoots Etoile with the silver bullet. Etoile dies, changing back into a man while Christine looks on in grief.