In 1876, immediately after the death of General George Armstrong Custer at Little Big Horn, a government stagecoach crossing the remote southwest desert is robbed of its payroll and its driver is killed. Capt. Nathan Brittles, who oversees the Seventh Cavalry at nearby Fort Stark, is disturbed to learn that the attackers were southern Cheyenne Indians, as he knows that the Cheyenne rarely venture so far south. Later, while thinking aloud by his wife Mary's grave, Nathan, who is retiring from the Army in six days, realizes that his last mission will be to drive the Cheyenne back north. The next day, however, Nathan's commander, Major "Mac" Allshard, orders him to take his wife Abby and genteel niece, Olivia Dandridge, along with the Indian patrol and deliver them to Sudros Wells, where they are to catch a stagecoach East. After registering a formal protest with Allshard, Nathan leads the large patrol from the fort, with Olivia and Abby in tow. Upon seeing the pretty Olivia with a yellow ribbon in her hair, rival suitors Lt. Clint Cohill and Lt. Ross Pennell each wonder if she is wearing the symbolic ribbon in their honor, but she refuses to reveal her preference. Soon after leaving the fort, Nathan hears from his scout, the southern Sgt. Tyree, that two white men have been spotted riding toward Sudros Wells. Tyree later reports that a large group of Arapaho Indians are also traveling toward Sudros Wells. Concerned for the women's safety, Nathan orders his patrol to take a different, slightly longer route to the depot, fully aware that the detour may mean missing the stage. Along the new route, the patrol is surprised to see a large herd of grazing buffalo. Tyree and Nathan speculate that an inter-tribal council meeting is being planned, at which the white men, Indian agent Karl Rynders and his interpreter, will be selling guns, and Red Shirt, an ambitious, radical Arapaho chief, will be claiming responsibility for the return of the buffalo. In order to protect the depot residents, Nathan orders Tyree to go to Paradise River and bring the small unit he commands there to Sudros Wells. Before Tyree reaches Paradise River, however, his unit is attacked by Indians, who are then chased off by Nathan's men. As a storm breaks over the desert, Tyree's patrol rides on and wounded Corp. Mike Quayne is operated on by Dr. O'Laughlin. By the time the patrol reaches Sudros Wells, the outpost has been decimated by the Indians, and the patrol is forced to turn back for Fort Stark. That night, Clint, who has been quarreling with Ross over Olivia during the entire trip, spies on the council meeting with Nathan and Tyree and watches as Rynders and the interpreter are brutally murdered by the Indians. When the patrol reaches Paradise River the next day, Nathan orders Clint and three squadrons of men to stay behind and cover their crossing. After Nathan promises to return the next day to relieve Clint, Olivia kisses the lieutenant goodbye, finally indicating her romantic choice. Back at Fort Stark, Nathan declares the mission a failure and, despite his imminent retirement, asks permission to return to Paradise River. Allshard, however, insists that Ross, who will soon be second in charge under Clint, lead the men back to the river. Although Nathan agrees with Allshard that the lieutenants' mettle should be tested and proudly accepts a silver retirement watch from his troops, he later joins the patrol at the river. Noting that he is technically still in charge, Nathan orders Clint and the troops to remain at the river, while he and Tyree try to negotiate peace with the Indians. Despite a warm reception from Pony-That-Walks, an elderly chief, Nathan is unable to alter the Indians' drive toward war. Consequently, he orders his men to attack the Indian camp at midnight and scatter their horses. With no deaths or casualties, the raid is a success, and the Indians are forced to return to their reservations on foot. Just as Nathan is about to ride off for California, Tyree presents him with a letter from the War Department, promoting him to chief of scouts at a new post. The newly assigned lieutenant colonel accepts the congratulations of his fellow cavalrymen at Fort Stark, where a dance is in progress and, after blessing Clint and Olivia's engagement, leaves the party to share his good news with Mary.
