A week before the "Pioneer Days" celebration in Mesa City, which marks the town's fiftieth anniversary, Chaunce Ralston, the straitlaced son of the town's banker, disputes the claim of precocious Arnette Flynn about her grandfather Hercules' past heroics. Arnette contends that "Gramp" was the first sheriff of Mesa County and, before that, a bandit who held up the old "iron horse." As she and Chaunce argue aboard the train, Arnette pulls a lever, which causes the train to speed away. Meanwhile, Frank Bailey is on his way to town to run the newspaper, which his father owns, as the first step toward eventually taking over the business. Frank sees the screaming children on the runaway train and jumps aboard. He stops the train, and when they see the eastbound express heading for them, Frank runs the train in reverse back to town, where Arnette's young friend Zero switches the tracks just in time to avoid an accident. Later, when Arnette sees her schoolteacher, Ruth Morris, approach, she enlists Frank's help to keep Ruth from telling Gramp that she has been playing "hookey." Attracted to Ruth, Frank is happy to accept the assignment. That night, Zero overhears Chaunce's egotistical and blustering father, Edward E. Ralston, agree to arrange for two men from Reno, Blackie Morgan and Lutz, to run an old-fashioned dance-hall and gambling dive during the celebration for a cut of the take. The next day, Arnette convinces Frank to give her a job as a reporter, and she has Zero tell Frank what he heard. Later, as Ralston makes a speech in city hall, Arnette shoots spitballs and hits the cymbal which Chaunce plays. The continual interruptions cause the hall to explode in laughter. Frank nominates Gramp, who has been feuding with Ralston for forty years, to play the sheriff in the reenactment of the holdup of the "iron horse." When Ralston finds his car covered with "Flynn for Sheriff" stickers, he says that the town is not big enough for himself and Gramp. Happy to hear the words, Gramp challenges Ralston to a duel, and Chaunce accepts for his cowardly father. Ruth and Frank talk Gramp out of going through with the duel for Arnette's sake, although Gramp is worried about what she will think of him if he backs off. The next morning, Ralston nervously awaits Gramp, until word is sent that Gramp is drunk at the new saloon. Arnette leads the drunken Gramp home, amidst jeers and laughter. That night, Arnette, ashamed of her grandfather, awakens Zero in his treehouse to run away, but they spy Ruth's fiancé, Barton Henshaw, who is Ralston's nephew and a cashier at his bank, meeting with Morgan and Lutz. After overhearing Morgan, to whom Henshaw owes money, plan to really rob the bank during the supposed fake robbery the next day, Arnette returns and tells Gramp. Despite her pleas that this is his chance to prove he is a hero, Gramp says that he does not want to save Ralston's bank. The next day, during the robbery, when Henshaw says he can't go through with it, he is shot in the hand by one of the crooks. Gramp, seeing Arlette and Zero yelling from the stage on which the robbers have escaped, rides to the rescue. After the back wheels of the stage, which Arlette and Zero have loosened, fall off, Gramp captures the crooks, and he is acclaimed a hero by his proud granddaughter. Frank hugs Ruth, as Ralston attempts to apologize to Gramp. At Arlette's instigation, Ralston and Gramp shake hands, after which Chaunce kisses Arlette. Her initial grimace turns into laughter, and she slaps Chaunce on the shoulder.