U.S. marshal Whip Wilson is on his way to Blue River with his rowdy cohorts Ted Crosby and Texas, when they spot a stage being robbed. A shootout ensues, and after Whip kills one of the robbers, the others flee. The driver, Joyce Westbrook, tells them that the Blue River Stage Line, which is owned by her father, is being continually robbed. She is secretly meeting her father to deliver a shipment of money to help keep the business open and cannot understand how the bandits knew about her cargo. Before he leaves to meet Joyce, Westbrook explains to Sheriff Preston and lawyer Frederick Kingsley that he is seeking a U.S. postal contract to help restore his business and the public's confidence in it. Although Preston and Kingsley publicly expressed empathy for Westbrook, they are secretly in league with saloon keeper Reardon and a mysterious leader, "Mr. Blackwell," to ruin Westbrook's business so that they can buy the stage line. After Westbrook leaves, Kingsley orders Reardon to send two henchmen to the mine to kill Westbrook. When Joyce, Whip, Ted and Tex arrive at the secret location, they find Westbrook dead. After Joyce decides to run the business herself, Whip, Ted and Tex offer to help her apprehend the robbers. Later at the stage line office, Kingsley advises Joyce to sell out, suggesting that he knows a buyer who is prepared to buy the line at a discount, but Joyce declines. To secure a government contract, Joyce must meet the U.S. postal contract requirements of running the stage at least once a week with a passenger. At the post office, handicapped postmaster Perkins confides to Whip that he is suspicious of Kingsley and Preston. Meanwhile, Tex and Ted recognize two of the stage robbers at the saloon after which a fight between the four men ensues. Whip subdues one of the robbers with his whip and defeats the other in a fistfight. Whip then suggests that Tex and Ted concentrate on finding the robbers' ringleader. Later that week, Whip and Tex visit the stage line office where Joyce tells them that at Kingsley's suggestion, she sent out a stage carrying Ted as a passenger. Suspecting Kingsley is involved in the robberies, Whip and Tex quickly depart and catch up to the stage just as Kingsley's men ambush it. After Ted is wounded, Whip and Tex chase off the bandits and race back to the stage. Whip takes Ted back to Blue River on horseback, while Tex remains behind to complete the stage run. Later in Blue River, Tex overhears Kingsley order Reardon to send Yarrow and several other men to Needle Canyon in an hour. As Kingsley meets the men and orders them to kill Whip, Tex and Whip eavesdrop nearby. When their presence is detected, Whip and Tex flee with the henchmen in pursuit. After losing the men on the trail, Whip and Tex find Kingsley and demand to know who "Blackwell" is. Before he can reveal the truth; however, Kingsley is killed by an unidentified rifleman. Back at the saloon, Reardon tells Preston that he and "Blackwell" will continue to plot against the stage line. Later at the post office, Joyce receives word that the postal inspector will meet her in Greenville that afternoon to finalize the postal contract. After Joyce leaves for Greenville, Whip rushes to catch her, but Reardon's henchmen are waiting at Robles Rock pass to ambush her. Although the henchmen shoot the driver, Whip arrives just in time to stop the runaway stage and chases away the bandits. Joyce and Whip return to town with the postal inspector and finally secure the contract. Knowing that only someone with access to the post office would have knowledge of the meeting with the postal inspector, Whip discovers that Preston has keys to the post office. When Whip confronts Preston at his office, he notes that Preston's boots are covered in Robles Rock clay. After Perkins corroborates that Preston has been surreptiously entering the post office, Whip surmises that Preston is actually "Blackwell" and arrests him and his gang. Ted, now fully recovered, considers running for sheriff and reveals his romantic affection for Joyce, who then decides to stay in Blue River with him. When Whip decides to continue his travels to Spring Valley, Tex, intrigued by the promise of further adventures, joins him on the trail.
