The life of peaceful rancher John Benedict (William Holden) is torn apart when his family is massacred by a gang of marauding outlaws and his farm is destroyed. He assembles a team of mean, lawless convicts to act as his posse as he pursues the gang responsible for the deaths of his loved ones.

Following the Civil War, rancher John Benedict returns home to Colorado from a trading expedition with his partner Free State. Greeting him warmly is his wife Della and their two sons and two daughters, as well as Sheriff Whitcomb and state militiaman Lt. Abel Lawson. Lawson hopes to offer John's oldest son Morgan a place at West Point Military Academy, stating that Morgan can bypass senatorial confirmation because of John's Congressional Medal of Honor, bestowed upon him for his valor during the war. Although John is loathe to gain privilege from his medal, Free insists that he present the option to Morgan. At lunch, John privately asks the boy if he would like to attend West Point, and despite his enthusiasm, Morgan declines, unwilling to leave John and Free to work the ranch alone. John seems to agree but then announces to Lawson that Morgan will be matriculating, as the Academy needs more men like him. Later, John is hunting on the mountain when he sees a band of Comanche Indians racing away from the ranch. John runs home to find his family slaughtered and the horses stolen. A mortally wounded Free gasps for John to look for Morgan in the barn, and when John finds his beloved son inside hanged from the rafter, he begins to boil over with sorrow and rage. Free whispers that the Indians were led by a white man with one white eye, then dies. Hours later, Whit arrives with a posse and a captive, a Comanchero, or Hispanic trader. With Whit's approval, John pistol-whips the man into confessing his leader's name, Tarp, and destination: the Mexican outlaw hideout Pueblo Plata. John sets out to track down Tarp, and although the posse accompanies him, by Texas they turn back, promising to sell his cattle and wire him the earnings. John crosses into Mexico and, knowing he needs protection to confront Tarp, stops at a prison camp called "the Hole" to recruit henchmen. The commandant's interpreter, Cholo, informs John that the commandant's price to hire prisoners is too high, but John is unconcerned about the cost. Against Cholo's recommendation, John chooses six men: Job, a runaway slave who killed his captors; Frenchman Quiberon, who tries daily to escape up the mountain walls surrounding the prison; a ferocious German named Zweig who is chained like an animal; William P. Hoop, a craven American criminal; Chamaco, a Mexican paid assassin; and Cholo himself. They set out on the trail with armed guards, but John soon has the men tie up and disarm the guards and explains his mission. He promises the men their freedom in exchange for their support against Tarp. Although each man grumbles skeptically, they are won over by John's faith in their word. After outfitting his gang, John cuts off Job's chains, and along the trip further into Mexico, Chamaco eagerly asks about John's family and states that all he knows of his father is that he had blue eyes. Once in Pueblo Plata, the group offers the Indian guards liquor in exchange for access to Tarp. As John and his men approach, Tarp shoots at them, but John forces a terrified Hoop to claim that they are there to trade guns for horses, so Tarp allows them closer. Once in sight, John's men begin shooting the Indian guards, and a battle erupts. While killing the guards, John's men cover him as he climbs the rock wall to Tarp's room, but Tarp manages to flee before John can reach him. Downhearted, John thanks the men and heads off alone to track Tarp. First Chamaco, then Job leap on their horses to accompany him, and one by one the others follow suit, trailed by Hoop, who is afraid to stay behind alone. Over the next five months they travel through the desert, often wreaking havoc in the towns through which they pass. In one town, John encounters Whit, who is now a U.S. Marshal collecting a prisoner. Noting John's gang and newfound disregard for the law, Whit declares that he no longer knows him and states, "Morgan would run from you." Envisioning his son's dead body, John heads to the local bar alone and downs a bottle of whiskey. Soon after, the gang finds him. Chamaco, noting his revered leader's despair, reveals that he believes John is his true father, but John replies, "Get the hell away from me, you bastard." Chamaco, burning with sorrow and resentment, steps away, then suddenly turns and shoots John in the chest. The other men draw around and silently pay their respects. Days later, John regains consciousness in the bed of local nurse Elizabeth Reilly, who informs him that the bullet landed an inch from his heart. She offers to send a letter to his family, and when he roughly refuses, assumes he is cold-hearted. Later, Whit visits but John refuses to see him, and without John's knowledge the lawman explains John's background to Elizabeth. Later, she urges John to return to ranching and tells him about her grandfather, who stated that a heart filled with vengeance will rot. Over the weeks of his convalescence, John grows attracted to Elizabeth's warmth and intelligence, and when he is ready to leave she asks him to stay. He kisses her passionately but then, still consumed by his lust for revenge, turns to leave. His search for Tarp soon leads him to another small town, where the labor camp commandant recognizes him and drags him off to The Hole. Meanwhile, in Silverton, a drunken Hoop enters the saloon Chamaco now owns and reveals that he has heard that John is imprisoned. Chamaco sets off immediately, dragging Hoop along. He reunites the gang to free John, and in a harrowing escapade, they pull their leader to safety over the prison wall. Facing Chamaco, John declares: "You missed my heart by one inch--any son of mine can learn to shoot better than that." The next day, John, having learned that Tarp is being held for trial at a nearby U.S. Army camp, bids the gang goodbye. Knowing John is too proud to ask for help, all six ride out to join him, happily firing their guns in salute. Together, they ride the trail to the army camp, which is on the verge of attack from a group of Comanches hoping to free Tarp. John's gang sneaks into the camp, where the wounded Lt. Mercer warns John at gunpoint not to attack Tarp. Although Mercer soon collapses, allowing John access to the barn where Tarp is being held, John hesitates and instead decides to help the soldiers defend the camp. Although Hoop is disgusted, Chamaco vows to stay with John and the others agree. With the weakened lieutenant's eventual blessing, John takes control of the defense, planting sticks of dynamite around the camp. When the Indians attack, although they outnumber the soldiers, the exploding dynamite decimates them and they soon retreat. After the battle, John hears Chamaco call out to him. The young man has been fatally wounded, and dies in John's arms. Each gang member mourns quietly, then John strides to Tarp's barn but as soon as he has his enemy in his sights, walks away. The gang members are confused, but John declares that his heart has been rotten for too long, and rides off alone. When he turns to give one last gunshot salute, the gang responds in kind.