In 1876, during the Great Sioux Wars, U.S. Cavalry Capt. Phillip Donlin serves at Fort Abraham Lincoln in the Montana Territory. His duties take him away from his wife, Celie Donlin, who feels neglected and unhappy. She is having an affair with Lt. John Haywood, whom she tries to persuade to resign the army and run away with her. Unknown to them, her husband overhears their conversation and confronts them. Heartbroken, he leaves the next morning on another patrol. His latest mission is to arrive at Camp Yellowstone and warn General Custer and his meager forces, numbering a few hundred men, of the large buildup of Sioux forces gathering at the Little Big Horn River. During a stop at a watering hole, Capt. Donlin's patrol is reached by Lt. Haywood's unit and told to return to Fort Lincoln, but the captain disregards the orders and decides to attempt to warn General Custer, whose forces had already left for Little Big Horn River.