The Flaming Frontier.

Bob Langdon, a young Pony Express rider, is given an appointment to West Point, but is forced to leave the academy as the result of political intrigue stirred up by enemies of his friend, General George A. Custer. Bob returns to the west and is made a scout for Custer's 7th Cavalry. At the Battle of Little Big Horn, Custer sends Bob with a message for aid, and Bob becomes the only survivor of the battle.