In the city of Crockett in the territory of New Mexico, Artie, a spoiled boy who lives at the Bar 20 Ranch with his aunt Amanda and uncle Buck Peters, refuses to go to school on the first day, in part because the old schoolmarm, Snooksie, disciplines the students with a switch. The foreman of the ranch, Hopalong Cassidy, orders Artie to go to school and warns him that the West will not always be what it is today. When the students go on strike against Snooksie and tie her up, Mayor Jud Thorpe, who is really rustler Dan Rawley, calls on Hoppy to restore order while his men raid the Bar 20 herd. Hoppy returns to the ranch and finds one of his ranch hands dead and begins to suspect Thorpe. Although Thorpe sends for a strict schoolmaster from Santa Fe to replace Snooksie, Hoppy asks the authorities for a gentle schoolmarm, and pretty June Lake arrives from Boston. Aunt Amanda invites her to stay at the Bar 20, and she quickly wins the affection of Artie, who encourages his schoolmates to reform. The ranch hands are distracted from building a new corral by the presence of a woman on the ranch, and instead refurbish the schoolhouse for her first day. When she discovers that all of the children but Artie have played hookie, the cowboys round up the students like a herd of cattle. Although both of Hoppy's friends, Lucky Jenkins and old Windy Halliday, are interested in June, Hoppy's mind is on work, and the three of them leave for the outskirts of the ranch lands, where they guard the herd. While they are away, Thorpe courts June. When Buck is ready to run the herd into Santa Fe to be slaughtered, he orders Hoppy to move them out of the mountains into pasture, where they are again vulnerable to an attack by Thorpe's men. The night before graduation, Hoppy, Lucky and Windy return to the ranch and learn that June is leaving for Boston after the ceremony. Although both Lucky and Windy propose to her, she tells Hoppy she is going to marry Thorpe. When Hoppy warns her against Thorpe, June accuses Hoppy of being too insensitive to understand love. Thorpe then hands out the diplomas while his rustlers wait for him to jump the Bar 20 herd. When the schoolhouse empties, Buster Sutton, an insubordinate rustler, enters to kill Thorpe and calls him Rawley. Thorpe shoots Sutton first and escapes, but June tells Hoppy that Thorpe is Rawley. The Bar 20 men head out to chase the rustlers as Thorpe joins them. A shootout and chase ensue. Hoppy goes after Thorpe himself and knocks him out while the rustlers surrender to Buck and his men. June promises to return in September and Hoppy nearly admits he will miss her, then walks back to the ranch house with Lucky and Windy.