In 1876, Johnny Jameson (Dan Dailey), a "drummer" (Door-to-door), is the only passenger on the inaugural run of the Tomahawk and Western Railroad's narrow gauge train through the Colorado Rockies (mountains). During the ride, the conductor tells Johnny that certain people, stagecoach operators for example, would like to see the railroad's franchise fail. Soon afterwards, Dakota (Rory Calhoun), Trancas and Gila, who work for Colonel Dawson, the area stageline operator, cause a giant boulder to fall directly in the path of the train. Engineer Terence Sweeny (Walter Brennan) manages to stop the train in time, and he and the crew then disembark to move the rock.