A sabotage ring is terrorizing Texas, and the dastards are destroying warehouses with explosives. Texas Governor Shuville (Russell Hicks of "Captain America") summons Roy Rogers and Frog Millhouse about an incident that occurred when they pursued the Wilson gang to Arkansas. They confronted the outlaws in a saloon that straddled the border between Arkansas and Texas. One of them is Duke Wilson (Stuart Hamblen), and Duke robbed the rodeo where Roy worked. Predictably, the Arkansas sheriff (Herbert Heyes of "New York Confidential") isn't happy about Roy's decision and lodges a formal complaint with Governor Shuville. "Rogers," Shuville observes, "I like the way you work. I like your nerve. I can use a fast-thinking man like you. In fact, I need you for an assignment." The Governor wants Roy to infiltrate a group that has thwarted each of his operatives. He wants Roy because "the fellows after you wouldn't mistake you for anything but a cowboy." The Governor briefs Roy, "There is a wave of sabotage sweeping through this territory with the precision of a well-organized gang. All my efforts to apprehend them have failed." The Governor warns Roy that his predecessor died making his report by telephone. The only available clues are the words: "following Merry." Roy volunteers to take care of the problem for the Governor. The chief executive asks Roy to memorize a secret phone number. This will be the only way they will communicate until they close the case. The Governor is the only man who knows about Roy’s mission. This became a familiar but suspenseful ruse in mystery-thrillers. The only man who can vouch for the discredited hero is a law enforcement official of the highest authority who winds up indisposed.
What the Governor has no way of knowing is that his personal secretary, William Kraley (Lloyd Corrigan of "Son of Paleface"), is the leader of the saboteurs. A suspicious Kraley warns Maurice (Gerald Mohr of "The Angry Red Planet") and his henchmen to be on the look-out for Roy Rogers. He explains that Roy Rogers quit the rodeo after a private conference with the Governor. Kraley tells them their new "key word: triplets."Later, while Roy is singing and strumming his guitar in the back of a cafe, two actresses from The Merry Makers Carnival and Tent Show enter and order the $35 cent lunch. Roy serves them coffee, and they have a conversation. "Every town we play has an explosion or a fire," Judy Mason (Peggy Moran of "Horror Island") points out. The other girl, Ruby Smith (Dorothea Kent of "Pin-Up Girl"), gripes because an explosion interrupted her dancing act. Roy says he is looking for a job, and Judy agrees to notify her stepbrother Dave (James Bush of "Massacre River") about Roy needing employment. The comments about the explosions arouse Roy's suspicions after the Ruby uses the words “following Merry.” He visits the carnival in the town of Rawhide with Frog. Roy decides to join the carnival to learn first-hand what is really happening behind the scenes. The carnival features a mind-reading act with a turban-clad individual known as Maurice the Mental Marvel. Actually, Maurice is Kraley's second-in-command. The act is rather sophisticated. A cowgirl with a microphone in her ring, Judy Mason, which she calls 'the mystic stone' asks spectators to confidentially address their problems to the stone. Interestingly, the stone serves as a microphone. A man behind the stage, usually David Mason, sits at a radio transmitter and relays the information to a man in a turban and a robe. Roy and Frog disrupt Maurice's show and their commotion prompts Mason to give Roy a chance to sing. Later, after he lies about a date with Judy, Roy has a run-in with Maurice, and they tie him up and abandon him in a warehouse about to be blown to smithereens. Afterward, Roy contacts Shuville, and the Governor and his men cruise into a trap because Kraley listened in on their conversation. They think they are about to have a head-on collision with another vehicle when in fact all that is happening is they are driving toward a huge mirror. This is rather ingenious. The Governor's car tumbles down the side of a mountain.
Meantime, Roy suspects that they have trapped the saboteur when Frog and he take over the mind-reading act. Maurice catches Roy using the receiver. When he is about to shoot Roy, Mason threatens to expose them all. Instead of Roy, Maurice shoots Dave in the back. Maurice frames Roy for Dave's murder. During his stay in jail, Roy learns that the Governor isn't expected to survive the car crash. Roy telephones Frog, who appears in a disguise masquerading as Roy's grandfather, and tells him to tell Judy about Maurice's treachery. Judy and Ruby arrange for Roy to escape from jail and returns to the carnival. Kraley passes along information during the Maurice act. Maurice tries to intervene, and Roy guns down Kraley. Roy and the Sons of the Pioneers hightail it on horseback to prevent the saboteurs from dynamiting a metal bridge that a train will cross. Roy climbs the bridge and defuses the dynamite. While he is climbing on the bridge, Maurice's henchmen keep him busy dodging their lead and firing back at them.
"King of the Cowboys" is a predictable but entertaining, lightweight espionage western. Roy Rogers makes a charismatic, clean-cut hero in a white Stetson. He warbles several tolerable songs. Veteran director Joe Kane keeps the action constantly moving headlong so this B-movie doesn't wear out its welcome during its 56-minute running length.