Future leader of the human resistance John Connor is the reason Skynet has sent 3 Terminators back to stop him fulfilling his destiny.
Before John Connor is born to Sarah Connor and Kyle Reese, his mother Sarah is the target of an assassination by a Model T-800 Terminator, in order to prevent the birth of John. His father dies before he is born and when John is a young child he is taken to live with a foster family. And Sarah is sent to the Peskadero Mental Hospital where she is under the care of Dr. Peter Silberman.
As a young teenager, John Connor is the target of another assassination attempt. This time by a more advanced model, T-1000. Along with the assassin travelling back in time, another T-800 model Terminator is sent to protect John at all costs. John finds it imperative that he saves his mother, against the Terminator's advice. From the hospital, the Terminator drives John and Sarah towards Mexico.
John, his mother, and the Terminator stop at a friend of Sarah's for a new car and supplies. After a long day of trying to fix a broken Bronco, John and the Terminator begin to bond. They realize that Sarah is missing and deduce from a carving she made on a picnic table, that she is trying to change the future by killing Miles Dyson. John and the Terminator stop Sarah in time to come up with a plan with Dyson to stop Judgement Day from occurring.
The four of them break into the Cyberdyne Systems lab to destroy the Terminator arm and CPU found in the printing press over a decade ago. The T-1000 follows them there and chases them from the lab to a steel mill he is thrown into a vat of molten steel. John throws the arm and the CPU from the lab into the vat. The T-800 has John and Sarah lower him into the vat to make sure that all technology from present or future Cyberdyne Systems or Skynet survives.
After his mother's death in 1997, John lives off the grid in Los Angeles. Believing that Judgement Day has been stopped, John tries to live his life as normally as possibly, until another Terminator, series T-X, was sent back in time to kill his future lieutenants in the Human resistance. The Human resistance sends back a T-850 model of the Terminator 101 to ensure the survival of John Connor and Kate Brewster.
The T-850 saves John and Kate from the T-X's initial attack at the veterinarian office that Kate works at. The terminator drives Kate and John to the grave of Sarah Connor. It turns out the the coffin that Sarah would have been in is filled with guns that she left behind just in case Judgement Day was not stopped. The T-X and the police converge on the cemetery and the T-850 steals a hearse to escape.
The terminator heads to a safe location that he is pre-programmed to take them to to keep them alive. John thinks that he can stop Skynet from going online and orders the T-850 to take both of them to advert the activation of Skynet. They arrive too late and are unable to deactivate Skynet. John asks the dying General where the core to Skynet is so that he can shut it down and the General is instructed to go to Crystal Peak, a military base built into the Sierra Nevada mountains.
The T-X takes control of the T-850 and reprograms it to kill John and Kate, but cannot when John reminds him that his mission is to ensure the survival of John Connor and Kate Brewster. When they arrive at Crystal Peak, the T-X attempts to kill them and the T-850 stops her by detonating his hydrogen fuel cell. John and Kate find out that Crystal peak was not the core of Skynet but a Cold-War era nuclear fallout shelter.
In 2018, John Connor leads the Human Resistance attack on the Skynet base. He finds human survivors and schematics for a new type of Terminator that incorporated living human tissue. He returns to the Resistance's headquarters located on a nuclear submarine. While he was gone, the Resistance discovered a radio frequency that they believe can send an order to shut down Skynet machines. They plan an offensive attck on the Skynet base in San Francisco in four days time. This is in response to an intercepted message from Skynet that included a "kill list" that listed his name seconded by Kyle Reese's.
On his travel to San Francisco he is met by Kyle Reese and his mute friend, Star. John finds out that they were travelling with a cyborg, named Marcus, after he is hit by a magnetic landmine and assumes that he was sent to kill him. The cyborg truly believes himself to be human. John orders him destroyed but Marcus escapes and John chases after him. In the pursuit,Marcus saves John's life from Skynet hydrobots and John spares his life if he helps the Resistance get into the Skynet base.
John, Marcus, and a team of Resistance fighters sneak into the Skynet base. When Marcus interfaces with the computer to access the inner section of the base, he discovers that he was built by Skynet to lure John Connor into the base and kill him. And the radio signal the Resistance discovered was given to them by Skynet in order to track down and destroy the Resistance's headquarters.
Marcus tears out the hardware linking him to Skynet and continues to assist John in destroying the base and fighting a new T-800 model 101 Terminator. John is mortally wounded but succeeds in blowing up the base as he, Marcus, Kyle, and Star are air lifted out of there. Kate tries to save John but his heart is too damaged and Marcus offers his own heart to John. John recovers and radios to the Resistance fighters that the battle is won but the war is far from over.