In 1877 Colorado Territory, a young woman Cresta Lee and a young Colorado Private Honus Gent are drawn together by fate when their group is massacred by the Cheyenne. Cresta, who has lived with the Cheyenne for two years, regards Honus with distrust and derision, calling him "Soldier Blue." Yet the two must press forward to Fort Reunion, where Cresta's fiancé, an army officer, waits.
The pair push through a harsh, sun-scorched desert with scant supplies, hiding from Native American riders and keeping to the shadows. They are spotted by Kiowa horsemen, forcing a tense test of nerve and resolve. Under Cresta's insistence, Honus defends himself and wounds the Kiowa chief when challenged; yet the chief’s men knife him, leaving Honus and Cresta to move forward as the only survivors. The rift between them widens as Honus—almost puritanical in his sense of duty—struggles to understand Cresta’s attitude toward the Indigenous people and the war around them.
A dangerous pursuit follows, led in part by a corrupt trader who sells guns to the Cheyenne; Honus manages to destroy the latest shipment, a small victory that heightens the trader’s vendetta. An injured Honus finds himself in a cave where Cresta leaves to seek help, while Cresta herself hurries toward Fort Reunion. There she learns that her fiancé’s cavalry unit plans to attack a peaceful Cheyenne village the next day. She rides out to warn Spotted Wolf, the Cheyenne chief, Spotted Wolf. The chief, under a U.S. flag, rides out to extend a hand of friendship, hoping for peace, but the soldiers follow orders and unleash artillery on the village.
The cavalry charge decimates the Cheyenne, and the men who survive the initial onslaught face brutal violence as some soldiers rape and kill Cheyenne women. Honus tries to halt the atrocities, but his efforts come to nothing and he is arrested for treason by his own comrades. Cresta, determined to protect the vulnerable, leads the remaining women and children toward safety, but her group is discovered and massacred. Cresta survives the massacre and is arrested for treason as well. Meanwhile, Honus is dragged away chained behind an army wagon, and Cresta is left to endure the aftermath with the few Cheyenne survivors.
