Dejected by the defeat of the Confederacy, O'Meara (Rod Steiger) heads west toward Sioux territory. After surviving a test called the "Run of the Arrow," he is accepted as one of the Oglala Sioux people and marries Yellow Moccasin (Sara Montiel). The couple create a family together and live contentedly until Capt. Clark (Brian Keith) arrives with a group of cavalry, hoping to build a fort nearby. O'Meara acts as a scout for the troops until their wary peace is shattered.