Frontier Marshal Jim Lane and his partner Sierra Pete must find the culprit behind the attacks on the Overland Mail. Pony Express riders are being murdered and Congress is about to take away the contract from Tom Gilbert. Although the attacks are the work of a local Indian tribe, Marshal Lane suspects that another sinister force is behind their actions. He arrives in La Paz and befriends Gilbert, as well as his beautiful daughter Helen, but all of their lives are plunged into danger as the Indian attacks increase. Sabotage, kidnapping, cattle stampedes, booby-trapped bridges - the Indians will stop at nothing to destroy the mail service. At first, no one suspects that behind their deadly acts is Gilbert's friend and confidante, Frank Chadwick, but as the violence builds and the corpses pile up, Lane is determined to uncover the mastermind and make the Overland Mail safe once and for all.