The mystery begins when the intrepid German detective Carl Lemberg seeks help from his famous rival, Nat Ridley, terrified by the murders of his brother and uncle in Mexico. Both men were killed by the same brutal method: a dagger thrust to the heart, accompanied by a card bearing the symbol of a double dagger from the Tola society. Soon after, Lemberg's investigator Dan Steele is murdered similarly, and Lemberg himself is killed in a taxicab in New York. Taking the case to avenge his friend Steele, Ridley uncovers a bloody struggle over highly lucrative oil wells originally bought from Mexican owners who had vanished.
Realizing the Tolas want the oil-rich land back to re-establish their ancient power, Ridley travels to Texas and crosses into Mexico disguised as a hardware salesman. Along the way, he encounters dangerous adversaries, including the ruthless El Capitan Martolo and Don Castro, who stop at nothing to regain control of the territory. Ridley also rescues Cora Ardell, a cousin of the Lembergs, who was kidnapped and tortured to force her to sign over the property rights.
Pushed into a corner by relentless pursuers, Ridley and his loyal cowboy companions use ingenuity and grit to survive narrow escapes, including a daring rope descent down a sheer cliff. Finally, Ridley uses a brilliant deception, disguising himself as a deaf mute to infiltrate the Tola headquarters. In a dramatic showdown at a building straddling the international border, the trap springs shut. El Capitan and Don Castro are defeated in a fierce firefight, the surviving conspirators are jailed, and the terror of the Tola gang is broken once and for all.
