Colonel Mackenzie, the commander of the 4th Cavalry Regiment at Fort Clark, Texas in 1873, receives secret order from President U.S. Grant to stop Mexican bandits from crossing the Rio Grande River into the U.S. or from returning to Mexico.
In order to trap a band of outlaws who have been raiding ammunition wagons being sent to the Mexican army, Colonel Mackenzie uses a heliograph, a signaling device that uses mirrors, to coordinate his troops ambush on the marauders.
After an extensive campaign, Colonel Mackenzie and his men manage to capture the leader of a ruthless outlaw band. Their victory is short-lived, however, because the bandit quickly escapes from the guardhouse taking the daughter of a visiting general as captive with him. Mackenzie must cross the Mexican border, sneak into the outlaws' camp, rescue the hostage and return to Fort Clark before the general realizes that his daughter has been taken captive.
Jed Pierce, an infamous outlaw, is sentenced to be hung. Pierce's brother, Ben, kidnaps an important Mexican emissary on his way to Washington on a secret mission and sends word to Mackenzie that unless his brother is released, the diplomat will be killed. Believing that Ben will kill the diplomat and whoever escorts Jed to the prisoner exchange site, Mackenzie plans to turn the tables on the outlaws.
Mackenzie's attack on a band of marauding Indians is cut short when he receives word that a troop of Mexican federales are riding to the sound of the shooting. As his men return across the Rio Grande, Mackenzie has the misfortune of bumping into a colonel from the Inspector General's office who arrests him for crossing the Mexican border.
While pursuing a gang of outlaws, Colonel Mackenzie enters a remote farmhouse and finds a woman bludgeoned to death and Andy Wheeler barely conscious from what he claims as a pistol whipping. Wheeler explains that the outlaws who attacked him and the dead woman fled when Mackenzie's men appeared, but the Colonel doubts Wheeler's story and plots to expose him and trap the rest of his gang in the process.
Colonel Mackenzie's troops are ordered to prevent renegades from filtering across the border into Mexico and, as a result, Dubov's supply wagons are being attacked in areas that they formerly patrolled. Mackenzie suspects that the wagons are carrying rifles intended to arm a counterrevolutionary force to retake Mexico by France and he sets a trap hoping to capture the guns and disperse the nascent military force before they can organize.
A cholera epidemic strikes western Texas and half the regiment stationed at Fort Clark are suffering from the disease, including the post doctor who dies while treating the troopers. A desperate plea for medical supplies is answered, but the wagon is ambushed and the morphine stolen. Although Colonel Mackenzie has contracted the cholera himself, he insists on leading the patrol to locate and recapture the desperately needed medicine.
Colonel Mackenzie tries to negotiate a peace treaty with the notorious Comanche chief, Quanah Parker, while the agreement he negotiated early with Chief Running Bear falls apart when his son is shot while hunting a mountain lion.
Colonel Mackenzie discovers that the son of an Apache chief he has captured is actually a white boy taken from his family long ago. The youngster's aunt and uncle agree to take the child as there own, but Mackenzie soon realizes the "brave" would be much happier with his adoptive Indian parents and schemes to return the boy to the tribe.
A flood changes the course of the Rio Grande putting the home of a rancher on the Mexican side of the river. The hot-headed owner ignores Col. Mackenzie's advice to vacate the property until Washington can work out a solution with the Mexican government. In the meantime a gang of outlaws raid the ranch forcing Mackenzie and his trooper to choose between avoiding an international incident or crossing the Rio and attempting a rescue.
A gambler and his conniving wife dream of creating the biggest ranch in the Southwest. Their tactics are just a bit illegal - they murder ranchers then buy their land at auction at rock bottom prices by scaring off other prospective bidders. The ranchers near Twin Forks appeal to Mackenzie for assistance.
Col. Mackenzie and his troopers hope to reduce conflict between white and Indians by supporting an Indian Department program to teach a band of Comanches to become farmers. A bigoted rancher who wants the land for himself tries to drive the Indians off their land by using dynamite.
While pursuing marauders on the Mexican side of the Rio Grande, Col. Mackenzie is severely wounded in an ambush. He manages to escape back to the homesteads dotting Texas' Chuma Valley, where the ranchers hide him from the outlaws seeking to finish him off. Mackenzie recovers sufficiently to lead the Texans in an inspired counterattack against the brigands.
When a former American general attempts to overthrow the Mexican government, Colonel Mackenzie is ordered to strike deep into Mexico, capture the traitor and return home without engaging the Mexican army.
Colonel Mackenzie is ordered to rescue a young mother from a bandit's haven in Mexico so she can testify before a committee of the U.S. Senate. Pretending to be a bandit, Mackenzie crosses the Rio Grande. Once there, he convinces the woman to return to the United States by the simple expedient of telling her that her son is living at Fort Clark, but first they must elude the bandit's cutthroats.
When a murderous band of Apaches kill a rancher's son, the bitter old man hires a gang of gunmen to exact revenge. Unable to find the marauders, the "posse" attack, kill and scalp a band of peaceful Commanches threatening an all-out Indian war. Colonel Mackenzie and his men must capture the murderers before the Commanches go on the warpath.
Troopers are being murdered by a court-martial-ed ex-soldier and his gang with a psychotic vendetta against Mackenzie and the army. Known as "The Hawk", he seeks to kidnap his son from his former wife, so the family is taken to the fort, but the boy actually worships his father and his crimes.
A headline-hungry newspaper reporter from Washington travels to Fort Clark intent on pillorying the U.S. Army's attempts to bring peace to Southwest Texas just as Colonel Mackenzie is about to lead his men on another raid into Mexican territory. When the reporter follows him he's captured by the gang of outlaws and Mackenzie is forced to lead his men in a desperate rescue attempt.
When a rancher imports a herd of short-horn cattle onto the Texas range near Fort Clark, his neighbor, a cattle baron who runs only longhorns tries to drive him out - first by persuasion, then by running off his stock, then poisoning his waterholes and finally by murdering his men. Mackenzie sets a trap for the crooked rancher by pretending to be an Eastern reporter who is in possession of incriminating photographs.
When a bandit strikes across the border to rob American stagecoaches, Colonel Mackenzie crosses into Mexico in hot pursuit. His attack is not without casualties and the Colonel accidentally leaves one his men behind. Mackenzie must find the trooper before the U.S. Cavalry's unsanctioned raid is discovered by the Mexican authorities.
By murder and intimidation, a crooked lawyer and his hired guns plan to take over Brackettville, the nearest town to Fort Clark and Colonel Mackenzie's Fourth Cavalry Regiment, insisting that the troopers remain outside the town's limits. The lawyer hopes to lure Mackenzie into gunfight with one of his gunslingers, but the Colonel counters by forcing a showdown on his own terms.
Three of Mackenzie's troopers disappear while on patrol. When Mackenzie refuses an offer from a cattle rustler to trade the men's loves for safe passage across the Rio Grande with their stolen beef, his men mutiny, not realizing that the cavalrymen have already been slain and outlaws are wearing their uniforms pretending to be hostages.
The fate of Mackenzie's Raiders hangs in the balance when one of their members is captured south of the Rio Grande while attempting to rescue his injured brother. The trooper is court-martialed and if he saves his neck by testifying he was under orders to cross into Mexican territory, it would mean the end of Colonel Mackenzie's career and the start of a war between the U.S. and Mexico.
Joe Topanga returns to Fort Clark as the first member of his tribe to receive a formal education. Upon his return, he suffers prejudice from the white and ridicule from his fellow tribesman. Duped by a former friend into joining his band of renegade Indians, Joe accidentally takes part in a robbery and is falsely accused of murder. Colonel Mackenzie attempts to clear Joe's name and bring the real killer to justice.
During the worst drought in memory grips Southwest Texas, ranchers and farmers desperately search for water to keep their animals from dying and their crops from withering. As the only man with a waterhole that hasn't dried up, Edward Beaudine is in a position to demand almost anything he wants from his neighbors. Not satisfied with charging exorbitant rates, Beaudined is determined to use his new-found power to create an empire by buying up all the abandoned ranches in the region.
After learning of Crazy Horse and the Sioux's victory over General George Armstrong Custer's 7th Cavalry Regiment, Comanche war chief Black Hawk decides to attack the 4th Cavalry and kill its hated commander, Colonel Mackenzie. Rather than chase the renegades with his small force, Mackenzie decides to lure the Comanches into a trap using an empty Fort Clark as bait.
Mackenzie and his men arrest Grimes, a deserter who stole a wagon full of repeating rifles which he intended to sell to the Apaches. The trooper escapes from the guardhouse and heads for Mexico with Mackenzie and a patrol in hot pursuit. Grimes schemes with the Apaches to lure Mackenzie into a trap, capture him and hold him as a hostage to trade for the stolen rifles.
An outlaw gang steals an army wagon full of repeating rifles and hightails it for Mexico. Mackenzie and his men cross the Rio Grande and steal the wagon back. Trying to hide the wagon's contents from the Mexican federales, Mackenzie drives the wagon towards the U.S. alone, but is captured by a Mexican colonel. When the outlaws kidnap the Mexican colonel's daughter and offer to trade her life for the rifles, Mackenzie offers to help the colonel rescue the young woman and wipe out the gang in the process.
A wealthy woman learns of Mackenzie's secret dealings with Mexican patriots supporting Porfirio Díaz and attempts to blackmailing him into marrying her. Her plan backfires when the identity of the woman she sends to meet with the rebels is discovered and Mackenzie is accused of undermining the forces for democracy in Mexico.
Although six months of searching has failed to turn up any evidence regarding two troopers who disappeared during a raid in Mexico, Colonel Mackenzie is convinced they are still alive. Trooper Williams, one of the missing soldiers, manages to escape from the silver mine where he has been held captive and is rescued, but suffering from amnesia, Williams can't retrace his steps to lead the Fourth Cavalry to the other missing soldier.
Colonel Mackenzie and his men capture three outlaws after a desperate gunfight that wounds half the troopers and most of their horses. In order to return to Fort Davis, they must walk across 150 miles of drought-scorched territory while the remaining bandits wait for an opportunity to pounce, rescue their men and wipe out the small patrol.
Disatified with the Fourth Cavalry's success at combating cattle rustlers, the Matthews family hires Sam Bates as a scout to track the outlaws, not realizing that Bates is, in fact, the gang's leader. After Bates leads the Matthews boys into a deadly ambush, he sets his sights on bigger game - leading Mackenzie himself into a trap, not realizing that the Colonel has set a trap of his own.
Howard Duncan, a disgraced army officer, drummed out of the cavalry by Colonel Mackenzie for an unprovoked attack upon a peaceful Indian village, organizes a band of renegades to avenge himself upon Mackenzie and his former regiment. After Duncan wipes out patrols sent to deal with his band of cutthroats, Mackenzie decides to take lead a picked band of his raiders to eliminate the threat to his command.