During the American Civil War, Union cavalry officer Major Amos Dundee is relieved of his command for a tactical error at the Battle of Gettysburg 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 Apache war chief Sierra Charriba, Dundee sends his scout Samuel Potts to locate Charriba and begins raising his own, unauthorized force. He attempts to recruit Confederate prisoners led by his former friend, Captain Ben Tyreen. Tyreen bears a grudge against Dundee and refuses his request. Before the war, Dundee had cast the deciding vote in Tyreen's court-martial from the United States Army for participating in a duel, leading to Tyreen's dismissal. Dundee is a Southerner, but fought for the Union.