In 1998, three years after successfully stopping the threat of Skynet, Sarah Connor (Linda Hamilton), once the future leader of the Human Resistance against machines, has become a battle-hardened figure isolated in her grief following her son’s death. She now dedicates her life to hunting and destroying Terminators to avert the impending Judgment Day and the war that looms ahead. However, despite crippling the Cyberdyne Systems company, she learns that the military merely experienced a setback in their agenda to create autonomous weapons, leading to the rise of Legion, Skynet's future successor. Struggling with her past encounters with machines, she finds herself at odds with Carl (Arnold Schwarzenegger) and Grace (Mackenzie Davis).
As a consequence of altering timelines, Sarah discovers that Dani Ramos (Natalia Reyes), a young woman destined to lead a new resistance against Legion, requires her guidance just as she once trained her own son, John Connor. Meanwhile, John and Sarah are enjoying a quiet moment at a beach in Livingston, Guatemala, when tragedy strikes as they are attacked by a T-800 Terminator, an aging model created by Skynet. This particular Terminator, having completed its original mission, gained autonomy and integrated into society, even marrying a human and raising a stepson. Despite Sarah's animosity towards Carl for her son’s demise, Dani begins to see Carl’s efforts to redeem himself, forming a bond with him as she recognizes his well-meaning intent.
Fast forward twenty-two years, a new advanced model known as Rev-9 is dispatched to Mexico City to eliminate Daniella "Dani" Ramos, who is working in an automotive plant alongside her brother Diego (Diego Boneta). The duo aspires to emigrate to the United States in pursuit of better opportunities for their ailing father. Dani’s childhood was marred by tragedy, including her mother's abandonment and subsequent death while attempting to cross the border. Initial belief leads Sarah to think Dani is the fated mother of another Resistance leader, only to eventually realize that Dani herself is destined to embody a role similar to John's in the future fight against Legion.
As events unfold, Grace arrives from the future, a cyborg soldier, part of Dani’s mission to shield her younger self from imminent danger. Struggling with her own humanity and cyborg status creates tension, particularly with Sarah, who views her as merely a machine. However, Grace understands the importance of their mission and trusts Carl, the T-800, seeing the complexity of its evolution beyond the malevolent machines they once faced.
The Rev-9, adept at infiltration with its ability to assume different forms, confronts Dani and her allies. After a series of harrowing encounters, where the Rev-9 kills Diego and corners Grace and Dani, the old T-800 steps in, using explosives to temporarily disable the formidable adversary. They regroup at a motel, where Sarah shares she receives alerts about incoming Terminators—each ending with “For John.” As they form plans to combat the Rev-9, tensions rise, and necessity brings everyone to disclose their past pains.
Realizing they must locate a military-grade EMP, the group embarks on a high-stakes mission, prompting them to escape in a stolen C-5 Galaxy as the Rev-9 pursues. In a stunning revelation during their dangerous flight, Grace confides in the group about the critical need for Dani to step into her role as the Resistance's leader. As they plummet into chaos, the team stands to confront the Rev-9 in a hydro-power plant, where a fierce battle ensues, resulting in devastating injuries.
In a climactic moment, Dani faces the Rev-9 directly as Grace sacrifices herself, urging Dani to harness her power core to deliver the final blow. Amidst the intense struggle, Carl and the Rev-9 grapple in a last-ditch effort, culminating in Carl’s poignant farewell to Sarah—“For John”—before an explosion obliterates them both.
Sometime later, Sarah and Dani, now more bonded than ever, watch over a ten-year-old Grace at a park, determined to change the course of the future. With unyielding resolve, Sarah tells Dani that she must prepare for the challenges ahead, as Grace observes them driving off, a poignant symbol of hope amidst the tumult of their intertwined destinies.
