Luke Perry is assigned to deliver a Gatling gun to a distant army post. Colonel Martinez discovers that Luke's stagecoach will not have a cavalry escort and determine to steal the gun and force the young driver to deliver it to Juarez's forces fighting Emperor Maximilian. When the American mercenaries assisting Martinez learn that Maximilian has been overthrown they decide to make use of the Gatling's firepower to rob an express office.
—David Bassler