In Aug 1876, Black Cloud and his elusive renegade Comanches, the only Indians not at peace with the white inhabitants of the Southwest, attack a scouting patrol seeking the Comanche chief as it stops for water at the desert town of Dry Buttes. During the night, Black Cloud sends a stampede of horses into the town, followed by overwhelming numbers of Comanches. The town is burned to the ground, and all but six cavalrymen are killed. These survivors, led by flinty Sgt. Matt Trainor, head into the desert for the one-hundred-mile trek back to Fort Macklin. Not long after their departure, they encounter a stagecoach driven by Romany O'Rattigan and carrying three passengers--argumentative whiskey salesman Henry Ruppert, former scout Prophet Satterlee, and elegant Julia Lanning, the sister of Fort Macklin's commander, Maj. Lanning. The threat of Comanche attack unites the two parties, and the thirsty soldiers are relieved to see that O'Rattigan carries a large water barrel on the coach. When a group of Comanches later attacks the coach, however, the water barrel is shot full of holes. Trainor takes the offensive and the Indians retreat, but Trainor now believes that the only safe route to the fort is through the hills. Because this route will add miles to the trip, they decide to take Satterlee's advice and visit an abandoned trading post in search of water. On the way, they encounter a man who identifies himself as a cattle buyer but whom trooper Jim Starbuck recognizes as a murderer named Denver Kinnaird. Trainor places Kinnaird under arrest, and the party moves on to the trading post. There they learn, to their dismay, that the well Satterlee described has gone dry. Buried in the sand are quantities of carefully wrapped guns that look just like the ones Black Cloud has been using, and Trainor realizes that the weapons were placed there by the gunrunner who has been supplying the Comanches. Not knowing what else to do, Trainor rations the rest of the water. While on night watch, young soldier Billy Creel worries that they will all die soon. His friend, Rusty Potter, tries to distract him with stories, but to no avail. The next day, the group comes upon a Kiowa boy named Little Knife, who attends a reservation school but was captured by Black Cloud while hunting. Little Knife, having chewed through his bindings and escaped, asserts that Black Cloud hates all Indians who are at peace with the white man, but Trainor distrusts the boy and leaves him behind. Julia argues that the boy deserves at least a drink, but Trainor relents only after seeing him running to catch up with the stagecoach. An old soldier named Floyd, who was injured in the Dry Buttes battle, passes out from lack of water, but there is none left, and he dies. Little Knife then remembers that during dry spells, his people got water at an old mission some miles away. At the mission, Little Knife locates the well, but finds that the water only drips from its source. The party catches the precious drops in buckets, but it takes hours to fill each container. The travelers take cover among the ruins of the mission as two Comanches approach. The soldiers capture them, and one of the Indians admits that the rest of the Comanches under Black Cloud are on their way to the mission to find water. Seeing a way to stop Black Cloud for good, the sergeant suggests that they send a messenger to the fort for troops, while they stall Black Cloud at the mission. Starbuck considers this plan suicidal, but the group nonetheless votes to attempt it. Because of his small size, Little Knife is given a horse and sent to Fort Macklin. When the Indians arrive, there are so many of them that Trainor is forced to light the dynamite the men have placed around the mission. The explosions are forceful enough to drive the attackers off, but trooper Martinez is killed. Little Knife, meanwhile, rides until his horse drops and then continues the journey on foot. At the mission, Black Cloud meets Trainor under a flag of truce, but because the chief will not surrender his guns, the sergeant refuses to give the Indians the many gallons of water he claims they have stocked at the mission. During another battle that night, Ruppert, who has done little else but complain, sacrifices his life to save Satterlee. The next day, O'Rattigan, dressed in his finest clothes, again tries to offer Black Cloud water in exchange for Comanche guns. Black Cloud rejects the offer, and O'Rattigan is shot in the back as he returns to the mission. Meanwhile, Little Knife, having long since run out of water, crawls slowly across the sand. The next night, as Julia watches over the wounded O'Rattigan, Starbuck sees a bill of sale in Kinnaird's gear and realizes that he is the gunrunner. Kinnaird runs from the mission, but Starbuck pursues and shoots him. The Comanches shoot Starbuck, and later, Rusty is killed while again trying to comfort Billy. Stunned, Billy pours the last of the water into the dead man's mouth, and just then, Black Cloud launches another attack. This time the Indians penetrate the mission, but as they begin to jump through windows and over walls, Maj. Lanning and his troops arrive from the fort. The cavalry finally defeats Black Cloud, after which Billy, Satterlee, Trainor, O'Rattigan, Julia and Little Knife are honored and thanked by the commander. Trainor declares that those who died gave their lives for something worthwhile.