While investigating shady dealings in stolen weapons, Tex and Kit Carson find a disquieting obsidian amulet. Then, in Presidio, while they are about to get a scoundrel involved in the shady deal to squeal, the Indio Pablito throws a poisonous arrow at the unfortunate one, transforming him into a dried up mummy: that is the terrible death reserved for those who betray the secret of the Sierra Encantada. In order to solve the mystery, it is necessary to turn to El Morisco, the arcane sciences scholar who lives in Pilares looked after by the devoted Eusebio.