The film begins with a prologue revealing the tumultuous past of Isaac LeMay**, depicted as a man who traversed the vast expanse of the Great Plains during the early days of American expansion. With a sordid history tied to both violence and the exploitation of women, LeMay fathered children with several prostitutes, drawing the ire of the Cheyenne people. Following a series of unforgivable acts, a Cheyenne chief by the name of White Thunder imposed a curse upon LeMay, foretelling that his own offspring would bring about his demise. Stricken with fear over this prophecy, LeMay embarks on a grim quest: to hunt down and eliminate his children before they have the chance to take his life.
As The Last Son unfolds, LeMay's journey leads him to a rundown saloon known as Two-One, where he plans to confront his son, Lionel, whom he fathered with the whore Anna, who also works at the establishment. In a violent encounter, LeMay fatally stabs the 20-year-old Lionel in the woods. Soon after, he seeks out Anna for information, discovering she is harboring a dark secret about another son. In a fit of rage and frustration, he viciously mutilates Anna, severing her fingers as he leaves her in despair.
The narrative further introduces Claire, another woman from LeMay's past, who has since moved on to a tranquil life in the woods with her family, including her daughter Megan. LeMay, perhaps showing some restraint, spares Megan, yet continues his violent rampage by murdering Anna’s husband.
Soon, the story shifts to Cal Talisker (Machine Gun Kelly), LeMay and Anna’s other son, who was abandoned at a home for boys to spare him from the life Anna despised. Growing up without parental support, Cal becomes an outlaw, embroiled in a life of crime and violence, robbing banks and killing law officers in a desperate quest for revenge against LeMay after the death of his twin.
After LeMay kills Lionel, Anna implores Cal to escape to Brooks, Canada, to save himself. However, stubborn like his father, Cal chooses to continue the cycle of violence. Meanwhile, a skilled tracker named Solomon learns of the turmoil in the town after hearing about the brutal murder of army men and the unrest stirred by LeMay. This leads to a turbulent buildup where Cal’s band of outlaws becomes more emboldened, leading to violent confrontations.
The climax of the film features an intense showdown between LeMay and Cal, culminating in a series of brutal gunfights. During a tense standoff, LeMay, believing he is elusive from the curse, finds himself facing his own demise, yet irony plays a cruel twist when it is Megan, whom he deemed harmless, who ultimately pulls the trigger. Her actions fulfill the Cheyenne prophecy, bringing an end to his reign of violence, but raising the question of whether the curse was ever real.
In the aftermath, Megan leaves the town and starts anew, establishing a peaceful life in Carson City, Nevada. Despite her family’s dark legacy, she marries and bears three children, thus ensuring that some part of the LeMay bloodline continues, even though their patriarch tried to eradicate it. The film concludes with lingering questions about the validity of curses and destiny, as Megan determinedly moves on from the shadows of her family's past.
