"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 religion, who deserted to Mexico after encountering religious and ethnic prejudice in the U.S. Army during the Mexican-American War. The plot focuses on 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.
John Riley, an Irish immigrant soldier recruited in the American army during the Mexican-American War faces anti-immigrant and anti-Catholic bigotry from his fellow servicemen and defects to the Mexican army.