In the late 1800's, responding to complaints of lost and delayed mail, United States government postal inspector Rex Allen travels to San Francisco from Washington, D.C. to determine a more efficient way to deliver mail between San Diego and San Francisco. There he meets Roger McCall, who has a $200,000 government contract to ship mail between the two cities by clipper ship. Rex also meets Sam Sawyer and John Brockway, the feuding owners of two competing stage lines that deliver the mail locally. Suspecting that the mail could be delivered faster by overland stagecoach via the Iron Mountain Trail, Rex convinces Brockway to try a test run as a race against the Argonaut , McCall's clipper ship. Eavesdropping Sawyer and his daughter Nancy also decide to join the competition. Later, while Rex helps Brockway's assistant, Slim Pickens, with rounding up horses for the relay stations, McCall meets with Orrin, the captain of the Argonaut , to discuss how much money they stole from the recent load of mail. Fearing the loss of the government contract, McCall makes plans with Orrin to sabotage Brockway's efforts, and as Rex and Slim herd the horses toward town, McCall and Orrin create an avalanche with explosives that scatter the animals. When Rex and Slim later report to Brockway that all but twenty horses were lost, Brockway assumes Sawyer was the culprit. Then McCall sends Orrin to set fire to Brockway's stagecoach and barn, but Orrin and his helper are caught in the act by Brockway, whom they shoot and put inside the coach. Meanwhile, Rex is amused at the sight of Orrin's pet monkey Marie, who has unleashed herself and followed Orrin down the street, but the sound of a gunshot prompts Rex to head toward the barn. Sawyer, who lives nearby, is the first to respond to the noise and sees two men exiting the back of the barn. When Rex and others arrive, they put out the fire and find Brockway dead. The marshal takes reports, but McCall demands that Sawyer be arrested, as he was found in the barn with a gun. Remembering that Orrin's monkey was in the vicinity, Rex asks that the sailor also be questioned, but McCall says he shipped off at midnight for San Diego. After convincing the marshal to delay the trial for thirty days, Rex and Slim race toward San Diego in Brockway's coach, hoping to prove that the inland trail is faster and simultaneously catch up with Orrin and bring him back to testify. Meanwhile, McCall sends thugs to sabotage Rex's efforts, and in town, calls for the circuit judge and rouses public demand for an immediate trial. Seeing no reason to get Orrin's testimony, the judge finds Sawyer guilty and sentences him to hang in ten days. Nancy grabs a horse and races out to tell Rex and Slim, and when she catches up, continues to ride with them to San Diego. However, twenty miles outside of San Diego, they are ambushed by McCall's men. At first, Rex tries to outrun the thugs, but after learning that the bridge ahead is out, he unhitches the horses, and he, Slim and Nancy jump out of the moving coach. Although the outlaws herd the team horses away, Rex catches Nancy's horse and rides it to San Diego, after promising to send horses back to Slim and Nancy. In San Diego, Rex proceeds to a saloon and through a window sees that the bartender is shanghaiing men for the Argonaut by drugging their liquor. Rex enters the saloon, pretends to get drugged and is carried away. Meanwhile, after arriving in San Diego, Slim and Nancy spot Rex's horse near the saloon and Slim enters looking for Rex. After being rendered unconscious by the bartender's alcohol, Slim is awakened by Rex in the back room where other victims are being held. When Orrin arrives to collect the men and pay off the bartender, Slim and Rex beat up Orrin and get him to write a confession which explains McCall's involvement in government fraud and the murder of Brockway. With little time left before the hanging, Rex races toward San Francisco with the confession note. When he arrives, the noose is around Sawyer's neck, but Rex calls out to the marshal, barely saving Sawyer. McCall tries to escape, but Rex chases him to a barn, where they shoot it out until McCall is killed. Later, after the rest of the crooks are in jail, Rex rides off on the first leg of the new overland mail run, which will be called the Pony Express.