A train rolls into the Old West settlement of Flagstone, where a mysterious man known as Harmonica takes on a surprise ambush, swiftly dealing with three masked assailants dressed in dusters. Initially, Harmonica was poised to confront an outlaw named Frank, but it soon becomes clear that the attackers were affiliated with the notorious bandit Cheyenne. The true motives behind Harmonica's quest to find Frank remain shrouded in mystery.
In a cruel turn of events, Frank and his gang ruthlessly execute Brett McBain and his three children as they prepare for a celebratory gathering at their ranch, dubbed Sweetwater. Shortly after, a former prostitute named Jill McBain, who recently married Brett in New Orleans, arrives at Sweetwater only to be confronted with the horrific crime scene. Confusion mounts as Frank leaves behind incriminating evidence designed to implicate Cheyenne in the murders.
Amidst this chaos, Jill struggles to make sense of the motivation behind the brutal killings. Frank is revealed to be a hired gun for railway magnate Morton, who had instructed him to intimidate rather than kill McBain, as the latter sought to profit from building a much-needed watering station on his land—a strategic move knowing that the railroad from Flagstone would inevitably pass through. The plot thickens as Jill learns that, now a widow and the rightful owner of Sweetwater, she is entwined in a deadly game of greed and deception following McBain’s murder, which pits Morton against Frank, who has his own designs on the property.
Harmonica’s journey intertwines with Cheyenne's as the two encounter each other; Cheyenne, now a fugitive, insists that he did not send his men to harm Harmonica. In a surprising turn, Harmonica saves Jill from Frank’s henchmen and uncovers Morton's ties to Frank, but his discovery leads to his capture. Just when hope seems lost, Cheyenne comes to the rescue, teaming up with Harmonica to aid Jill in securing Sweetwater by utilizing stored materials to construct a station.
Despite a perilous situation where Jill must fend off unwanted advances from Frank, she finds herself thrust into a public auction for the land. Frank’s goons use intimidation tactics to suppress bids on the property, but when Harmonica shows up with Cheyenne and offers an impressive bid of $5,000—equal to the bounty on Cheyenne's head—it disrupts Frank's plans. Frustrated by his inability to outmaneuver Harmonica, Frank becomes suspicious about Harmonica’s motivations.
As tensions rise, it becomes evident that Morton is playing his own hand; he attempts to kill Frank by bribing his men, but Harmonica intervenes, revealing his complicated relationship with Frank, who still has unfinished business with Harmonica.
In a climactic showdown, Cheyenne and his gang confront Frank’s remaining forces in a dramatic gunfight aboard Morton's train. The confrontation leaves a trail of destruction, claiming everyone except for Cheyenne, who heads toward Sweetwater, where a final confrontation unfolds. There, Harmonica and Frank face off in an intense duel, with a haunting flashback illuminating Frank’s dark past—a boy who had been left hanging as Frank committed a horrific act, with the boy's harmonica serving as a chilling reminder.
Harmonica ultimately prevails, outdrawing Frank and forcing the harmonica into his mouth, which triggers a final, painful recollection before Frank breathes his last.
As Harmonica and Cheyenne depart from Sweetwater, tragedy strikes anew; Cheyenne succumbs to a gut wound sustained during the earlier skirmish. In a solemn farewell, Harmonica honors his fallen comrade by placing his body on Cheyenne's horse, riding off as Jill tends to the railroad workers in the background, her future uncertain yet markedly changed.
