On 9 April 1865, the final day of the Civil War, O'Meara, an Irish American serving in the Virginia Infantry, shoots a Union officer and carries his wounded body back to the Confederate field hospital at Appomattox. While there, he watches as General Robert E. Lee, having just surrendered to General Ulysses S. Grant, dejectedly leaves Appomattox Courthouse. The doctor removes the bullet, the last one fired during the war, from his patient and gives it to O'Meara's friends. O'Meara returns home, where he receives the bullet as a war trophy, but is bitter nonetheless. Even his mother is unable to persuade him to accept defeat at the hands of the North, and filled with hatred, he leaves Virginia. Aware that the Sioux Indians are engaged in a war against the United States government, O'Meara guides his horse toward Sioux territory in the West. On the way he meets an old Oglala scout named Walking Coyote, who claims that he is going home to die. Amused by O'Meara's fascination with the Sioux, Walking Coyote agrees to instruct him in Sioux customs and language. As they near Walking Coyote's tribal homeland, however, the two are captured by a group of rowdy warriors. Led by Crazy Wolf, the Indians are about to execute the men when Walking Coyote requests the "run of the arrow," a ritual in which the pursuers allow the prisoners a head start in a race for their lives. Explaining that no one has ever survived a run, Walking Coyote encourages O'Meara to exert himself, but shortly after the chase begins, the older man's heart gives out and he dies. O'Meara runs until he, too, collapses in exhaustion, but a group of Sioux women hides him from his pursuers. The next day, O'Meara presents himself to the local chief, Blue Buffalo, claiming that he has survived the run of the arrow. Crazy Wolf is baffled by the white man's escape but accepts Blue Buffalo's pronouncement that no Sioux may kill one who has survived the run. Yellow Moccasin, who hid O'Meara in her tent on that first night, nurses him back to health, and the two fall in love. Blue Buffalo agrees to accept O'Meara as a full member of the Sioux nation, allowing him both to retain his Christian religion and to marry Yellow Moccasin. The couple adopts Silent Tongue, Yellow Moccasin's mute and orphaned young companion, and the family lives happily for a time. When Sioux chief Red Cloud and Cavalry general Allen agree on terms by which the U.S. government may build a stronghold named Fort Lincoln, Red Cloud asks O'Meara to accompany the builders to the approved site. During the journey, a Yankee soldier sacrifices his life to save Silent Tongue from a pool of quicksand. Moved, O'Meara listens when the leader of the expedition, Captain Clark, remarks that Lee's surrender, rather than marking the South's demise, was "the birth of the United States." The group arrives at the appointed site, but as they begin construction, Crazy Wolf and his band of renegades attack, killing Clark. Realizing that Crazy Wolf is trying to start a war, O'Meara disarms him and gives him the opportunity to run for his life. Lieutenant Driscoll, the Indian-hating Yankee whom O'Meara had shot near Appomattox, interferes in the ritual, wounding Crazy Wolf with a bullet. O'Meara takes Crazy Wolf back to Blue Buffalo, while Driscoll, now in charge of the construction party, orders his men to build the fort in a more strategic site. Yellow Moccasin returns to the village to warn Blue Buffalo that construction is occurring outside of the agreed-upon corridor, whereupon the tribe prepares for war. Under a flag of truce, O'Meara orders the builders to return to the original site or lose their scalps. Driscoll injures O'Meara and orders his men to hang him for treason. At that moment, Blue Buffalo signals a large force to attack the expedition, and in the fierce battle that follows, most of the soldiers are killed. The Indians capture Driscoll, and as Crazy Wolf is skinning him alive for having violated the run of the arrow, O'Meara, unable to endure Driscoll's screams, kills him with the same bullet he used at Appomattox. Yellow Moccasin convinces O'Meara of his own allegiance to the U.S. flag, and together they accompany the surviving soldiers back to Fort Laramie.
