Tex and Carson protect the archeological expedition of the brothers Rick and Tony Roswell from the attacks of the hired ruffians engaged by an unknown enemy. The first suspect is Domingo Ortiz, a rogue with a sinister face, but when Tex and his pards enter his hovel to question him, they find him with his throat cut and next to him lies the corpse of his assassin! The mystery thickens, while the expedition escorted by the rangers reaches the Sierra Madre, where a submerged temple appears to hide an ancient Aztec treasure.