Law of the Badlands

Eight years before Custer's stand at Little Big Horn, an officer of the Seventh Cavalry was found guilty of murder, stripped of his commission, and remanded to the civil authorities. Jump ahead to the mid-1870s: a gang is robbing stages, murdering all on board, and letting Indians take the blame. Two people escape an attack, gambler Bob Tate and Karen, a teen. Tate protects Karen, leading them to safety. Not long after, the Sioux and Cheyenne prepare to attack Custer, and someone must get word to him. How does this connect to the old murder charge?