As he rides into Rancho San Angelo, a territory located both in Mexico and America, border investigator Roy Rogers is greeted by his friend, "Cookie" Bullfincher, the town's mayor and sheriff, and Cookie's friend, the padre. Roy has come to investigate a series of murders perpetrated at the nearby Monarch Silver Mine. While riding to the mine, Roy sees a man get shot while trying to escape from mine guards working for manager Rex Gridley and his partner, Gus Ulrich. When Roy reaches the body, he discovers a chunk of pure silver ore that was planted by Gridley and Ulrich. Later, at Rancho San Angelo, Cookie accuses Ulrich of murder, but he explains that Gridley gave orders to shoot any thieves found at the mine. Roy then receives a letter announcing that the famous writer of western novels, Lee Madison, is coming to town the next day. When she arrives on the bus, however, Lee is not recognized by Roy's men because they expect a man. Overhearing them say that Roy does not want Madison around, she tells them that her name is "Helen Clifford," and they give her a lift to the hotel. On the way, Roy ambushes the stage to prove to Madison that the real West is dangerous, but lets Lee go when he sees she is a woman. At the hotel, Lee asks Roy to read one of Madison's books, suggesting that it might give him some clues to his murder investigation. After Ulrich and his men leave to check the mine, Cookie follows and discovers that they are actually smuggling pure silver ore from the Mexican side of the mine to the American side, where it can be sold at high prices. When Ulrich and his men find a young Mexican man, Ignacio, snooping around the mine, they shoot him and hide his body in the mine. Soon after, Roy and Cookie go to check out the mine, and as they leave, Roy's horse Trigger gets a piece of silver ore stuck in his hoof. At San Angelo, meanwhile, English solicitor Lionel Bates inquires about a man called George Wallingford Lancaster, explaining that Scotland Yard is looking for him, a fact that terrifies Cookie. Bates also declares that the ranch at San Angelo soon will have a new owner. Later, Roy analyzes the ore found in Trigger's hoof and informs Cookie that it is mostly lead with only a trace of silver, whereas the ore found on the murdered man was pure. The next morning, Bates conducts a fox hunt, using an opossum instead of a fox. Gridley joins the hunt and he and his men pistol whip Bates. Soon after, Roy realizes that the pure silver is being smuggled into the American side of the mine and brings Lee and Cookie there to investigate. When they discover Ignacio's body, Roy sends Lee back to town, but Gridley kidnaps her. Seeing a riderless Trigger, the ranchers follow the horse back to the mine, where Gridley's men have started a shootout. After the ranchers capture most of the mine workers, Roy chases Gridley into the mountains, but stops when the mine owner threatens to kill Lee. Lee and Roy then re-enact the death scene on "page 77" of her novel Murder on the Border , which fools Gridley into thinking that Lee has been shot and allows Roy to knock him out. A few days after Gridley is arrested, Bates reveals that he has discovered that Cookie is really Lancaster and is the heir to the Rancho San Angelo territory. Lee then informs Roy that she plans to call her next book The Bells of San Angelo .
