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.

"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.