The governor of Texas sends for rodeo star Roy Rogers after learning of how Roy and his pal, Frog Millhouse, apprehended bank robber Duke Wilson and his gang. A wave of sabotage has been spreading through the Southwest, and the governor asks Roy to act as an undercover operative and find the culprits, who have been targeting factories supplying materials for the war effort. The governor's last agent, who was murdered, left a clue that he was "following Mary," and so Roy strikes out with Frog to look for Mary. Their travels lead them to the Merry Makers Carnival and Tent Show, which is owned by Dave Mason and features Maurice, who has a mind-reading act. Unknown to the governor, his assistant, William Kraly, is the leader of the sabotage ring, and it is he who orders Maurice and the reluctant Mason to carry out the bombings and arsons. Upon hearing Mason's pretty stepdaughter Judy comment on the unlucky incidents that seem to follow them, Roy decides to infiltrate the show and arranges to be hired as a singer. Kraly has already alerted Maurice and Mason that Roy is the governor's new operative, however, and they have hired him only to keep an eye on him. Kraly's method of operation is to send an agent to Maurice's show to pose as an audience member, and, while pretending to ask an innocent question, alert Maurice to his identity with a code word, followed by coded instructions. Roy uncovers this method, although he is still unaware of Kraly's complicity, and follows Maurice, Mason and their men as they go to blow up an important warehouse. They capture Roy, however, and after tying him up, leave him to be killed in the explosion. Roy escapes just before the dynamite goes off, and with Frog's help, learns that the ringleader will be going to the town of Freeman to pick up an important message. When Roy alerts the governor, Kraly finds out and sends his men to drive the governor's car off the road as he travels to Freeman. Meanwhile, Maurice catches Roy stealing his code book and is about to shoot him in cold blood when Mason stops him. Mason threatens to expose the operation but is shot by Maurice, who frames Roy for the murder. Roy is arrested but escapes with the help of Judy and her friend, magician Ruby Smith. The friends return to the carnival, where Kraly himself has come to deliver a message from the audience to Maurice. Roy shoots Kraly as he attempts to escape, while Frog captures Maurice and learns that his men have planted dynamite to blow up a supply train. Roy rides to the pass and arrives just in time to defuse the explosives as the train travels across the bridge. After the gang is rounded up, the governor presents a scroll of appreciation to Roy, "King of the Cowboys."