Jim Arnold asks the sharpshooter foreman of the Bar 20, Bill "Hopalong" Cassidy, and Red Connors to come to Wyoming to help him fight a mysterious rustler known as "Nevada," who has a Napoleonic complex. Arnold warns Hoppy's friend, Johnny Nelson, not to come along because Arnold's daughter Margaret, whom Johnny loves, has returned from school in Boston sophisticated and has fallen for Easterner George Perdue. Johnny leaves ahead of Hoppy to win back Margaret and finds Perdue, who owns the ranch adjacent to Arnold's, not only courting Margaret, but pushing Arnold to sell his ranch to him. Meanwhile, Nevada and his men steal Arnold's cattle before Red and Hoppy arrive by using a "Napoleonic strategy" of a divided attack. Cinco, a gang member, is wounded, and Nevada, who is really Perdue, treats him callously as he dies. Red joins Arnold, while Hoppy goes undercover as gambler Tex Riley and meets Windy, an old desert rat, who leads him to Nevada's Triple A ranch. When Tex says he is looking for Johnny Nelson to kill him, Nevada accepts him in his gang and leads him to Johnny, and Hoppy pretends to kill him. While trying to send a smoke signal to Arnold, Red and Johnny to warn them about Nevada's plans for attack the next day, Hoppy and Windy start a fire. Nevada and his men chase Windy and Hoppy, and Johnny arrives in the middle of a fight between them. After Hoppy saves Johnny from being kidnapped, they join Arnold's men and race to Nevada's ranch, where a gunfight ensues. There, one of Nevada's own men shoots him for his treatment of Cinco. Arnold then rounds up his stolen cattle, and when Windy finally realizes that Tex is Hoppy, Hoppy invites him to his Bar 20 ranch. Johnny promises to return to Arnold's ranch to marry Margaret.