One Man's Hero tells the little-known story of the "St. Patrick's Battalion" or "San Patricios," a group of mostly Irish and other immigrants of the Catholic faith who deserted to Mexico after encountering religious and ethnic prejudice in the U.S. Army during the Mexican-American War. The plot centers around the personal story of John Riley, an Irishman who had been a sergeant in the American Army who is commissioned as a captain in the Mexican army and commands the battalion, as he leads his men in battle and struggles with authorities on both sides of the border