It's 2018, fourteen years after a networked computer system became self-aware, determined humans were a threat, and nearly wiped out all of them through nuclear annihilation. Yet, bands of human resistance fighters exist, battling the ever-evolving robotic terminator machines built and programmed by Skynet to capture or kill the remaining humans.
While the official commander is General Ashdown (MICHAEL IRONSIDE), most of the resistance follows the orders of John Connor (CHRISTIAN BALE), the legendary leader prophesized to save humankind. After all, and in the future/past, his father, Kyle Reese, time-traveled back into the 1980s to warn John's mother of the pending disaster, only to have to protect her from a T-800 series Terminator sent back to kill her and her yet to be conceived child -- fathered by Reese -- who would one day grow up to become the resistance leader.
New to this bleak world is Marcus Wright (SAM WORTHINGTON), a convicted murderer who was executed in 2003 after being convinced by terminally ill Dr. Serena Kogan (HELENA BONHAM CARTER) to donate his body to a scientific corporation for research. After being rescued by teen resistance fighter Kyle Reese (ANTON YELCHIN) and his younger mute companion, Star (JADAGRACE BERRY), Marcus wants to meet John.
Following a violent encounter with Skynet forces where Kyle and Star are abducted along with others, Marcus encounters resistance pilot Blair Williams (MOON BLOODGOOD) who escorts him back to headquarters. But Connor and others, including Barnes (COMMON), are suspicious of Marcus after a surprising discovery about him.
From that point on, and as the resistance fighters prepare a new all-out assault on Skynet and the terminators, John must contend with this new presence, as well as trying to rescue his future father from a Skynet camp before General Ashdown's forces attack.