Upon reaching the mission settlement where he is to meet his wife, cavalry captain Victor Kaleb finds her barely alive after being tortured by Apache Indians. Kaleb shoots her to spare her further suffering, then rides after the perpetrators. Returning to Fort Bowie with the body of an Apache dressed in his wife’s clothes who he has apprehended and killed, the captain bitterly assails commander Maj. Brown for not offering the village church mission proper protection and allowing his wife to visit there unescorted. When Brown protests that the attack of the marauding Apaches was unanticipated, Kaleb denounces Brown as a coward and declares he is leaving the army. Brown warns Kaleb about the consequences of his action, but when Kaleb ignores him, Brown accuses him of being his wife’s actual murderer. Enraged, Kaleb shoots Brown in the arm and leg, then departs. Two years later, Gen. Miles arrives at Fort Bowie with several troops and visiting British officer Capt. Crawford, who is eager to learn of American military methods. Miles meets with Brown, who has been promoted to colonel, to demand an explanation of why the campaign against the Apaches continues to go poorly. Pointing out that Apache warrior leader Mangus Durango inhabits an area across the Mexican border known as the Devil’s Spine, Miles reveals his concern that the Apaches are in a position to wipe out both the cavalry and all friendly nearby natives. Brown declares that crossing the border without provocation would result in his court martial, but is surprised when Miles abruptly asks about Kaleb. Reading from Kaleb’s file, Brown relates that Kaleb was born in Yugoslavia and brought to America at a young age. After his parents’ deaths from cholera, Kaleb joined the army and served with great distinction with the Union forces during the Civil War. After deserting upon the death of his wife, Kaleb was court martialled in absentia and found guilty on seven counts, including the attempted murder of Brown. Brown reluctantly admits Kaleb has waged a very successful one-man campaign against the Apaches since deserting but he now suspects Kaleb is dead. When young Lt. Ferguson interrupts to inform Miles that Kaleb has recently been sighted, the general orders Ferguson to bring him to the fort. A few days later, Ferguson and his troopers return in haggard condition, having been plagued by Kaleb, who refused to return with them. Miles then learns that two cavalry scouts, Native American Natchai and Tattinger were friends with Kaleb and asks them to go in search of him. In the mountains beyond the fort, Kaleb and his German Shepherd dog soon discover the scouts. Kaleb refuses to go with the men, but after they successfully ambush a group of Apache stalking them, Tattinger is injured and Kaleb reluctantly agrees to help return him to the fort. Upon arriving at Fort Bowie, soldier Schmidt attempts to arrest Kaleb, but Miles intervenes and meets with the former captain privately. The following day, Brown refuses to believe Kaleb's revelation that the cavalry patrol outside of the fort has been lost to the Apaches. Miles then shocks Brown by informing him that Kaleb has been reinstated to his former rank to train a small group of men in Indian fighting tactics, thus preparing them to attack Durango in Mexico. Miles overrides Brown’s protests and reveals his intention to secure an official pardon for Kaleb as well. That afternoon, Kaleb tests the men’s endurance and attitude, selecting the hostile Schmidt, belligerent Irishman O’Toole, the deceptively docile reverend and explosives expert Reynolds, troublemaker Jackson, gun specialist Orozco and surgeon Scott. Crawford and Ferguson request permission to join the group and Kaleb agrees. The men depart Fort Bowie and soon come upon the remains of the patrol, whom they bury. Over the next few days in the hot desert, Kaleb drills the frequently antagonistic men in fighting tactics, turning basic objects like rocks, sand and tree branches into weapons. Crawford complains about the crudeness of these methods, but when Kaleb takes on Jackson and overpowers the armed, larger man easily without a gun or knife, Crawford reconsiders. The men also learn about explosives, Gatling guns and rock climbing. When O’Toole falls to his death from a steep mountain terrain similar to that of the treacherous Devil’s Spine, Kaleb advises the men that mistakes have deadly effects. One night, a scouting group of Apaches surround Kaleb and the men who having anticipated the ambush, kill all the Indians except for one who is tortured into revealing Durango’s plan to attack the cavalry the night of the next full moon. Kaleb returns to the fort to inform Miles, who approves the captain’s plan to climb the Devil’s Spine and make a surprise assault on Durango from the rear. Brown declares the mission cannot proceed as he has contacted Miles’ superior at Brigade headquarters, who is sending an investigator to look into the details of Kaleb’s reinstatement. Furious, Miles then orders Brown to accompany Kaleb’s outfit, which will depart at sunrise. Kaleb leads the men directly into Mexico, crossing the Rio Grande the first day. Upon arriving at the base of the Devil’s Spine, the men construct an elaborate crane in order to lift the horses, pack mules and gunnery to the mountaintop. At a gorge, the men construct a swinging bridge for the pack animals, but after a successful crossing, Jackson and Schmidt accidentally fall off the cliff. That night, the soldiers attack a group of Apache scouts and Ferguson is killed. The only Apache survivor is a young boy who Kaleb is about to kill, when Natchai stops him. The boy escapes and, grabbing a knife, fatally stabs Tattinger before fleeing. Knowing the boy will warn Durango, the men hurry to his encampment and find the Apaches in the midst of their war dance. The soldiers surround the Apaches and attack with dynamite and numerous Gatling guns. Kaleb confronts Durango, only to find his pistol is empty, but is saved by Orozco, who kills the Apache leader. A fatally wounded Reynolds survives long enough to light the fuse for a strategically placed explosive which detonates, setting off a rock slide that demolishes Durango’s camp. Returning to the fort with the handful of survivors that includes Brown, Kaleb learns from Miles that his request for a pardon has been denied, and consequently, the captain faces arrest and imprisonment. Brown declares this impossible, as he will report that Kaleb was killed at Devil’s Spine. The other men agree to corroborate the story, allowing the grateful Kaleb to disappear back into the desert and remain free.