In the year 1889, in the rugged landscape of Montana, a man emerges from a saloon, visibly wounded from a gunshot to the back. The lawman, Edward Johnson, steps in and instructs his partner, Lefty Brown, to take a detour to catch the assailant. However, the villain surprises Lefty by leaping from a window, and Lefty fails to apprehend him. Ed makes his way out to subdue the attacker and reprimands Lefty for his lapse in judgment. Without hesitation, Ed and Lefty march off to hang the man they have caught, dismissing Lefty's concerns about the legality of their actions, as Ed believes in his own form of justice.
Upon their return to the ranch, Ed and his wife, Laura Johnson, prepare for their upcoming move to Washington, following Ed’s recent election as Senator. Although Laura expresses her worries about leaving Lefty in charge of the ranch, Ed remains convinced of his partner's loyalty. The following day, they learn of rustled horses from a ranch hand, prompting Ed and Lefty to investigate. While Ed attempts to bequeath his rifle to Lefty, he is unexpectedly shot in the head by an unseen sniper. Lefty survives and rides back to the ranch with Ed's lifeless body. Overcome with grief, Laura blames Lefty for not safeguarding her husband and disregards his vow to avenge Ed’s death. Lefty buries the rifle and embarks on a quest for retribution.
Shortly after, Governor James Bierce and Marshal Tom Harrah arrive to provide solace to Laura. They reminisce about the days when Ed, Lefty, and Tom rode together as a trio. Learning that Lefty has set off to hunt down Ed's killer, Tom decides to find him to prevent any reckless actions. During his journey, Lefty encounters a young gunman named Jeremiah Perkins, who wishes to join him. Initially hesitant, Lefty takes the boy along after assessing his circumstances.
Tom eventually tracks down Lefty and Jeremiah. Confident about their mission, Lefty is disheartened to learn that Tom only intends to bring him back, not assist in capturing Ed's murderer. But upon hearing Jeremiah's enthusiasm and how he idolizes heroes from Western stories, Tom finds himself swayed to join their mission. As they separate for a lead that Lefty believes in, he stumbles upon the hideout of the gang responsible for Ed's killing. The trio devises a plan; while Jeremiah tends to their horses, Lefty and Tom position themselves for a confrontation. Just as they prepare to spring a trap, a man from the governor's camp suddenly intervenes, leading to a violent shootout that results in Jeremiah getting injured, and Lefty apprehending the killer. However, two gang members manage to escape.
After attending to Jeremiah’s injuries, Lefty begins to narrate a tale about Ed's heroics, but Jeremiah requests to hear the story of how Lefty's own leg got shot. Initially reluctant, Lefty shares his past encounter; he had gone after an outlaw without Ed’s backup, leading to an ambush. Caught in a box canyon, they were starved by the outlaws who waited outside for them to surrender. Lefty finally made a move—gaining his nickname "Lefty" after being shot while returning fire. Meanwhile, Tom grapples with interrogating Frank Baines, the captured killer, revealing the deeper conspiracy behind Edward's death. Tom burns the incriminating money that connected Frank to a governor's scheme and stumbles away drunk, leaving Lefty and the injured Jeremiah to bring Frank back.
As they make their way back, they are ambushed by the escaped gang members. Despite exhausting efforts, Frank manages to turn on them, and after a fierce struggle, Jeremiah secures a rifle and kills him, only to find himself gravely hurt. Lefty returns to the ranch, where the ranch hands mistake him for Ed’s murderer and attempt to lynch him. Laura intervenes but ultimately the hands tie Lefty up; she reads a telegram claiming Lefty killed Ed from Jimmy. Feeling utterly untrusted, Lefty breaks free and retrieves Ed's rifle, managing to gather medical supplies at gunpoint. He tends to Jeremiah's injuries and realizes he misjudged Jimmy all along. After mulling over their next steps, they decide to confront Jimmy in town.
Upon reaching the town, Jimmy is hosting a memorial for Ed. Laura implores him not to appoint Ed's senatorial runner-up, but Jimmy slyly references a shady railroad deal Ed opposed. Jeremiah sneaks into the gathering to gather information while Lefty seeks Tom's aid at the saloon. After a brief scuffle, Tom changes his mind and interrupts Jimmy’s eulogy, accusing him of orchestrating Ed's murder. Soldiers surround Tom but are soon embroiled in chaos when Lefty joins in to shoot. While rescuing Laura from Jimmy, Lefty learns the shocking truth from Jimmy himself: Ed was killed to clear the path for Jimmy’s railroad plans. As a result, Lefty knocks Jimmy out, but upon Laura's insistence to carry out Ed’s justice, she decides to handle the situation with her own hands, leading to Jimmy's execution.
In a moment of sacrifice, Lefty takes the responsibility to hang Jimmy himself, expressing concern that Laura might face consequences for his actions. He decides to ride off alone, leaving Jeremiah to be raised by Laura, thereby concluding the poignant tale titled The Ballad of Lefty Brown.
