A frontier scout, Jess Remsberg (James Garner), is searching for the murderer of his Comanche wife. All he knows is that it was done by a white man. While crossing the desert he rescues Ellen Grange (Bibi Andersson) from a pursuing band of Apaches, and returns her to her businessman husband, Willard Grange (Dennis Weaver). The couple has lived apart for most of the previous two years, since Ellen Grange was kidnapped by Apaches. She had been rescued, but then voluntarily returned to the Apaches to live with the son of the chief.
Jess learns from his friend, Lt. "Scotty" McAllister (Bill Travers), an experienced army sergeant anxious for a promotion, that the town marshal at Fort Concho has information about Jess's murdered wife.
Jess agrees to act as a scout for an Army cavalry unit of twenty-five inexperienced soldiers taking horses, ammunition and supplies to that fort. Willard and Ellen are joining them, as is horse breaker Toller (Sidney Poitier), a veteran of the 10th Cavalry (the "Buffalo Soldiers"). Taller has been contracted to provide horses to the army and will accompany the party, taming horses on the way.
The townsfolk treated Ellen Grange as an outcast after her first abduction, and are now even worse. Some men try to rape her, but she is rescued by Jess, aided by Toller. The two had previously had a confrontation in a bar, which, with McAllister's intervention, was resolved without ill feeling. On the morning the supply wagon is to set out it is learned she has again returned voluntarily to the Apaches. Her husband does nothing about it, but Jess goes after her. While rescuing her he discovers she has had a child by the now-dead son of the Apache chief, Chata (John Hoyt). Jess rides off with mother and child to catch up with the expedition.
The supply wagons, however, have been ambushed by Chata and his warriors, with serious losses of men, food and water. McAllister is seriously wounded, but is able to function. The men also look to Toller for leadership. Using the infant as a shield (he is Chata’s grandson), Jess and Ellen get through the Apache attackers encircling the besieged cavalry force.
McAllister devises a plan to break out of their position and take refuge in Diablo Canyon, where there is water and better cover. As part of the plan Jess is to speed to Fort Concho for help. That done, he can resume the search for his wife’s killer.
The plan succeeds. The besieged unit is able to break out and hole up in the canyon, and Jess, though his horse dies and he is parched from thirst, is able to kill his pursuers and get to the fort. Reinforcements are immediately sent to the canyon, where the unit is under constant attack and the defenders are being killed one by one. Willard Grange is captured by the Apaches and is tied upside down over an open flame overnight so that his cries of agony will prevent the others from resting.
At the fort Jess learns that the man he has been hunting for is none other than Willard Grange, who had been out for revenge for what had been done to his wife. Jess races to the canyon, arriving with the army reinforcements just in time to save the last few survivors (including Toller and Ellen Grange). Jess searches for Willard and finds him barely alive. Willard begs Jess to take pity on him and put him out of his misery, so Jess gives him his pistol and leaves. Moments later a single shot is heard.
The Apaches are disarmed and rounded up to be returned to the reservation. Chata is allowed a final embrace of his grandson before joining them. Jess, Toller and Ellen stand by the graves of the dead soldiers (including McAllister).James Garner as Jess Remsberg
Sidney Poitier as Toller
Bibi Andersson as Ellen Grange
Dennis Weaver as Willard Grange
Bill Travers as Lieutenant Scotty McAllister
Ralph Nelson as Colonel Foster (billed as Alf Elson)
John Hubbard as Major Novak (Fort Creel's C.O.)
John Hoyt as Chata
William Redfield as First Sergeant
Bill Hart as Corporal Harrington
Eddie Little Sky as Alchise
John Crawford as Clay Dean
Writing in The New York Times, Robert Alden gave a positive review, stating that Much of it is raw and ugly, yet it is a film that will grip you, a film that will have a shattering effect by the time you go back out into the street. A more critical, three-star review was written by Tony Sloman for The Radio Times, observing that it doesn't quite succeed, partly because the eclectic casting (including Swede Bibi Andersson and Brit Bill Travers) gets in the way of the authenticity.