El Puma, a Mexican desert guide, escorts an archaeological expedition headed by Professor Lucious Lloyd through the Indian badlands of Mexico. Marian, the professor;s niece accompanies the party as only she can translate the Aztec writings in the diary of her father, murdered on a similar expedition six years previous. THe professor is murdered by a knife, and the weapon is recognized as the property of El Puma. Magpie, a Federal Investigtor, knows that El Puma is really "Lightnin' Bill' Carson, a former federal agent who has been missing since Marian's father was slain. The reluctant Magpie believes that his old pal is guilty. Carson sets out to prove otherwise.