Le Meneur de Loups is set around 1780 in Dumas' native town of Villers-CotterĂŞts, and is supposedly based on a local folk-tale Dumas heard as a child. The story concerns Thibault, a shoe-maker, who is beaten by the gamekeeper of the Lord of Vez for interfering with the lord's hunting. Afterwards he encounters a huge wolf, walking on its hind legs like a man, who offers him vengeance; Thibault may wish harm on any person in return for one of his own hairs for each wish. To seal the agreement, the two exchange rings. As a result of this bargain he also finds himself able to command the local wolves, and hence gradually gains the reputation of being a werewolf.
Thibault's first two wishes kill the gamekeeper and injure the Lord of Vez. The wishes turn two hairs on his head long and red, as do his subsequent ones, which, though equally successful, also backfire against him in unexpected ways, leaving him scorned and hated by others in his community. Finally one of his wishes causes him to trade bodies with Lord Raoul of Vauparfond, who is having an affair with the wife of the Count de Mont-Gobert. Caught with the lady by the count as the result of an earlier wish against Lord Raoul, he is mortally wounded. He manages to keep himself alive until transferred back into his own body, only to find himself trapped in his own home, to which the townsfolk have set fire.
Escaping, Thibault takes to the forest, where he subsists on animals caught for him by his wolves and hunts and is hunted by the Lord of Vez. He has but one human hair left on his head. The conclusion of the book, however, brings him an unusual redemption.

Question: What occurs each time Thibault wishes for vengeance?
Answer: He grows a long red hair on the back of his head, trading bodies and backfire in other strange ways
Alternative answer: A hair on his head turns long and red.