During the American Civil War, Union cavalry officer Major Amos Dundee (Charlton Heston) has been relieved of his command for an unspecified tactical error at the Battle of Gettysburg (it is implied that he showed too much initiative) and sent to head a Prisoner of war camp in the New Mexico Territory. After a family of ranchers and a relief column of cavalry are massacred by an Apache war chief named Sierra Charriba (Michael Pate), Dundee, seizes the opportunity for glory, sends out his scout Samuel Potts (James Coburn) to locate Charriba and begins raising his own private army. He attempts to recruit Confederate prisoners led by his former friend turned rival from West Point, Captain Ben Tyreen (Richard Harris). Tyreen bears a grudge against Dundee and refuses his request. Before the war, Dundee cast the deciding vote in Tyreen's court-martial from the U.S. Army prior to the American Civil War for participating in a duel, leading to Tyreen's dismissal from the service and his later becoming an officer in the Confederate Army.