As the American Civil War intensifies, Union cavalry officer Major Amos Dundee (Charlton Heston) finds himself stripped of his command due to a perplexing tactical error at Gettysburg. With his reputation tarnished, he is banished to the New Mexico Territory, where he oversees a prisoner-of-war camp. However, his focus shifts as he seeks revenge against the Apache war chief Sierra Charriba following the brutal slaughter of a family of ranchers and a relief column.
Dundee initiates his quest for vengeance by sending his reliable scout, Samuel Potts (played by Richard Harris), to locate Charriba. At the same time, he assembles an unconventional team, gathering Confederate prisoners led by Captain Ben Tyreen (Richard Harris). Tension brews between Tyreen and Dundee due to a past conflict that resulted in Tyreen's dismissal from the Army, stemming from a pivotal vote during his court-martial after a duel.
As Dundee’s unauthorized cohort takes shape, it includes a motley assortment of individuals: a bugler named Tim Ryan, narrating the events and personally affected by Charriba’s violent raids; a crafty horse thief; a whiskey-loving mule-packer; a minister on a path for vengeance; and a determined group of black soldiers weary of menial duties. To lead them, Dundee reluctantly assigns Lieutenant Graham, who is both inexperienced yet earnest in his role.
Amidst the simmering tensions among the eclectic forces, they face several fierce clashes with Apache warriors, ultimately rescuing a number of young captives along the way. A rapid depletion of supplies pushes them to execute a daring raid on a French-held village, which reveals nothing but scarcity and despair. In the midst of this turmoil, Dundee grapples with his obligations to his men while feeling a burgeoning responsibility towards the starving Mexican villagers.
The situation becomes even more complicated with the arrival of Teresa Santiago (played by), a beautiful Austrian widow whose husband was executed for supporting Benito Juárez’s rebellion against Emperor Maximilian of Mexico. This incites competition between Dundee and Tyreen as they both vie for Teresa's affections. In a remarkable strategy, Dundee helps facilitate the escape of his French prisoners, who return to provide reinforcements, allowing him to mount a surprise offensive that secures much-needed supplies.
As camaraderie begins to blossom among Dundee’s men, a Confederate deserter named O.W. Hadley attempts to flee. In a critical decision, Dundee orders Hadley's execution; however, Tyreen intervenes by shooting Hadley himself to avert unrest within their ranks.
Throughout this turbulent campaign, Dundee's relentless drive for revenge against Charriba collides with challenges to his sense of loyalty and morality. A fleeting romance with Teresa ends abruptly when he is wounded by Apache forces, leaving him desperate for medical care in French-occupied Durango. His recovery leads to a tender relationship with a local woman, only to be shattered when Teresa discovers them together, resulting in resentment and heavy drinking as he tries to cope. Yet, Tyreen’s sharp reprimands and Graham’s initiative to stage a diversionary assault eventually restore his resolve.
With slim chances of tracking down the Apaches, Dundee devises a strategy to feign defeat and retreat to the United States. The Apaches, unsuspecting, walk into a carefully laid ambush where Ryan delivers a critical blow to Charriba. As Dundee and Tyreen plan their retribution, they encounter French forces who have established positions on the American side of the Rio Grande, leading to a fierce battle. In the midst of this, Tyreen heroically recovers their regimental colors but is gravely wounded, launching an individual attack against enemy cavalry that clears the path for his comrades.
As Dundee and his remaining men begin their retreat, it’s noted that the date is now April 19th, 1865. Unbeknownst to them, Lee has surrendered, the Civil War is over, and Abraham Lincoln’s assassination has occurred, leaving their fate hanging in the balance.
