Just after the Civil War, Texan Ken Morgan is the leader of a group of "bushwackers," Southerners who use armed resistance against the taxation policies of the Northern army occupying the area. One afternoon, Ken, who was a doctor before the war, aids Doris Conway, the daughter of Post commander Colonel Conway. Despite their initial attraction, when Ken and Doris realize that they are on opposite sides of the fight, Doris leaves in a huff. When Ken returns to the tavern run by his father, he finds Captain Hawkes and two other troopers demanding an exorbitant tax payment. As Ken and his men are chasing off the troopers, his friends, "Tucson" Smith, "Stony" Brooke and "Lullaby" Joslin, known as The Three Mesquiteers, ride by and aid one of the troopers, whom they believe is being unfairly attacked. Hawkes reaches the post, where he gives other collected taxes to Charles Bentley, the post's civilian attache. Unknown to Colonel Conway, Bentley and Hawkes have been squeezing money from the local residents and fomenting the war between the bushwackers and soldiers in order to cover up their own deeds. After the Mesquiteers arrive with the wounded trooper, they hear Hawkes tell Conway that Ken was to blame. Tucson, who grew up with Ken, does not believe Hawkes, but when he confronts Ken, Ken refuses to accept Tucson's help because he earlier had interceded on the trooper's behalf. Soon after, Ken and his brother Jimmy are about to be executed by Hawkes's men when the Mesquiteers rescue them. Jimmy is killed in the confusion, however, and Stony persuades Tucson and Lullaby to join the troopers so that they can obtain evidence of the corruption. The Mesquiteers soon realize that Conway is ignorant of Hawkes's activities, and inform him that Jimmy was shot in the back. Conway places Hawkes under arrest and goes to see Ken, but Bentley arranges for trooper Tompkins to shoot the colonel. Ken passes by as Conway is being murdered, and so Tompkins blames the killing on him. Bentley assumes command of the post and goes with the Mesquiteers to question Ken. Ken is wounded when he is captured, and Tucson and Stony are jailed for trying to help him. Lullaby sneaks into the post and passes a note to Doris that she must alert Major Briggs, the inspector general, of the current state of affairs before Ken, Tucson and Stony are executed. Doris then rides off to find Briggs while Lullaby slips his friends guns hidden inside a watermelon. As the Mesquiteers and Ken are shooting it out with Hawkes, Bentley and their men, Ken's father organizes the bushwackers and they join forces with Briggs's soldiers. The Mesquiteers and the reinforcements are victorious, and later Ken and his father are placed in charge of the district until a general election can be held.
