A sort of monstrous and bloodthirsty werewolf carries out massacres among the Apaches of the Sierras and only El Morisco, scholar of occult sciences in Pilares, can stop the curse! The pards, having run to the aid of their friend and of his devoted assistant, Eusebio, are attacked by Mitla, a diablera of Aztec ancestry, able to speak with animals and to incite packs of wolves to attack man. She is the witch who periodically changes her own brother, Guaimas, into a lycanthrope, stirring up his beastly ferociousness against the Indians.