In 2003, Doctor Serena Kogan of Cyberdyne Systems convinces death row inmate Marcus Wright to willingly donate his body for medical research before his execution by lethal injection. Fast forward one year, and the Skynet system perceives humanity as a threat and activates, leading to the catastrophic event known as "Judgment Day," which effectively wipes out a significant portion of the human population, an event chronicled in Terminator 3: Rise of the Machines. This marks the beginning of a relentless war between humans and machines, with Skynet creating Terminators to hunt down the remnants of humanity.
In 2018, John Connor emerges as a pivotal leader of the Resistance, launching a critical attack on a Skynet facility. During this mission, he uncovers human captives and plans for a new breed of Terminator that blends living tissue with machinery. Tragically, John finds himself as the only survivor of the operation after a nuclear explosion obliterates the base. However, Marcus miraculously survives the blast and makes his way toward Los Angeles.
Upon returning to the Resistance's headquarters aboard a nuclear submarine, John briefs General Ashdown (Michael Ironside) on his alarming discoveries. Meanwhile, the Resistance stumbles upon a radio frequency believed to be an "off switch" for the machines. With a plan to launch an offensive against Skynet's San Francisco hub within four days, they learn that John’s name is second on a kill list compiled by Skynet, right after a civilian named Kyle Reese. Unknown to the Resistance leaders, Kyle's future significance is crucial, as he is destined to become John's father.
As tensions rise, John meets with his wife Kate (Bryce Dallas Howard) and his subordinate Barnes (Common) to test their newfound signal on captured machines, successfully shutting them down. An inner conflict arises when Ashdown proposes an aggressive strike against the Skynet core, which is heavily guarded by human prisoners—a move John opposes, insisting on a rescue mission first.
Meanwhile, in the ravaged remnants of Los Angeles, Kyle Reese and his mute companion Star rescue Marcus from a relentless T-600 Terminator, clarifying the brutal reality of their ongoing war with Skynet. Inspired by John's message, they prioritize linking up with the Resistance, ultimately evading capture until an ambush leads to their captivity. In a desperate attempt to save them, Marcus proves to be incredibly resilient, escaping the destruction around him.
Resistance A-10 aircraft subsequently crash while trying to intercept a machine transport en route to Skynet's lair in San Francisco. During his journey, Marcus encounters downed pilot Blair Williams (Moon Bloodgood), and, after fending off human marauders, they press on toward John's base. However, just as Marcus is attempting to save the day, he gets wounded by a magnetic land mine, revealing an unsettling truth: he is part machine himself, housing a complex Bio-mechanical structure.
Initially deemed a threat by John, Marcus gains his trust by revealing Kyle's transport is heading to Skynet. Despite being considered expendable bait by Skynet, Marcus is determined to help John rescue Kyle and the other captives. Amidst escalating risks, John is stripped of his command but retains the loyalty of the Resistance fighters, who choose to follow his lead.
As the assault on Skynet's stronghold begins, Marcus hacks into the computer systems to disable defenses, allowing John to infiltrate and free the prisoners. However, the situation rapidly deteriorates as Skynet's machinations unfold. Just when it seems defeat is inevitable, Marcus rebels against his programming, aligning with John to battle a formidable T-800 model 101 Terminator. Consequently, John suffers devastating injuries, but in his final act of defiance, he triggers demolitions that obliterate the Skynet headquarters.
As they narrowly escape the explosive aftermath, Kate desperately attempts to revive John, but time is running out. In a poignant moment of sacrifice, Marcus offers his own heart to save John, ensuring his survival while sacrificing himself in the process. With the battle won but the war far from over, John re-establishes contact with remaining Resistance members, reminding them that the fight against Skynet continues.
