In late 1800s, as a wave of robberies and ruthless civilian killings sweep through the American West, the president appoints Tom Emery as the new territorial governor to resolve the lawlessness. U.S. Marshal Lash LaRue is on his way to escort Emery to Capital City, where he will assume his office, when local crime leader Ed West hears of the marshal's imminent arrival and orders his men to kill Lash. When three bandits mistakenly ambush a lone rancher, Lash witnesses the murder and realizes that the bandits were after him. Upon arriving at the stage depot to meet his partner, Fuzzy Q. Jones, Lash asks stage owner Charlie Jones why the outlaws are trying to kill him and not Emery. Jones explains that Emery, who lives nearby with his niece Betty-Jo, does not pose a threat until he reaches Capital City. Back at West's office, the bandits report that they have killed the wrong man. When the bandits recognize the stranger at the stage depot as the same man they spotted on the trail after the murder, West realizes that Lash is the marshal and orders his men to kill him on his way to Emery's ranch. As Lash and Fuzzy are riding to the ranch, Fuzzy's nose itches, signaling danger ahead, prompting the two lawmen to split up. Soon after, three bandits ambush Lash and a fight ensues. Fuzzy surprises Lash's assailants, and after shooting one, holds the other at gunpoint, while Lash challenges the remaining man to a "fair" fight. Lash prevails in the fistfight and then orders the bandits to leave the area. Once at Emery's ranch, Lash decides to put Emery on the regular stage early the next morning. Unknown to Lash, Emery's housekeeper, Clarke, who is pretending to be a deaf-mute, is working for West. Clarke reports to West, who stations several groups of his men along Emery's stage route to Capital City, to create a "thundering trail" of ambushes to inundate Lash with gunfire at several points along the trail. The next morning, Fuzzy, accompanied by a guard, is driving the stage when four masked bandits ambush him. Lash, who is scouting the territory ahead, hears the shots and rushes to the stage's rescue, but the guard is killed in the attack. Lash jumps on board the stage, shoots and kills two bandits and drives the others away. Although Lash warns Emery and Betty-Jo that there might be more violence, Emery remains determined to continue the trip. When three more masked bandits attack the stage, Lash advises Emery to return to the ranch and devise a new plan to reach Capital City. Meanwhile, West deduces that Lash will make another attempt at safe passage to Capital City along a different route and orders henchman Pete to round up the rest of the men waiting in ambush. Back at the ranch, as Clarke is serving refreshments, Lash lays out a plan to transport Emery in a covered wagon through the pass. Once Clarke finishes work, Lash reveals that he suspects Clarke of passing information to the leader of the bandits and so proceeds to change the plan. The next morning, as the covered wagon is driven through Boulder Pass, West's bandits attack the wagon and are met by an armed posse in the wagon. Cornered in the pass, the outlaws dismount and are chased on foot and shot by the posse. West's cohort Schaeffer is cornered by Lash, who uses his whip to disarm the criminal. A fistfight ensues and, while reaching for a boulder to crush Lash, Schaeffer loses his balance and falls from the cliff to his death. Days later, in Governor Emery's new Capital City office, Lash reports that West has been arrested and jailed, which has caused the outlaws to leave the territory. Emery adds that since the territory will soon be safe from outlaws, Lash might want to settle down, possibly with Betty-Jo, but Lash insists that Fuzzy's adventurous spirit keeps them both traveling. When Fuzzy suggests the governor appoint him permanently to the territory, however, Lash takes Betty-Jo in his arms and kisses her.
