Changing Flags is the true story of John Riley, a soldier who deserts the U.S. Army to fight for Mexico in 1846-48. His tale begins in Michigan when Riley launches a plan to afford passage home to Ireland: He enlists intending to desert in Texas, where U.S. troops are being sent to end a border dispute with Mexico. He anticipates becoming sergeant with higher pay. Denied promotion, he feels trapped. Then he is slipped a flyer offering cash, rank, and land to U.S. soldiers who join the Mexican army. That night, Riley crosses the river and gets caught in the relentless tide of history.