In 2029 AD, the human resistance captured one of SkyNet's Terminator infiltrators and had it reprogrammed to protect John Connor, their leader. They breached an enemy lab complex where they discovered SkyNet's time displacement equipment and managed to seize it, sending their cyborg warrior back in time to 1994, when John was in his youth. Upon arrival in Los Angeles in 1994, the Terminator entered a biker's club and demanded the clothes and bike of one of the patrons. When the bikers refused, a scuffle ensued with the bikers all being badly beaten, though not in any life-threatening way. One of the bikers surrendered his clothes and the key to his bike and the Terminator also took the bartender's shotgun after being threatened with it.
With clothes, transport and a weapon obtained, the Terminator set out to find the young John Connor. He found the boy's foster home, though John himself had rode off into town on his motor scooter. The T-800 tracked John to the Galeria Mall and found the boy about to be gunned down by a T-1000 disguised as a cop. The Terminator saved John and they evaded the SkyNet assassin, the machine then revealing its identity and mission. John wanted to return home but the T-800 advised against it, stating the T-1000 would likely try to reacquire him there. Nevertheless, John tried to warn his foster parents by finding a payphone, but when he called, the T-800 - mimicking John's voice - deduced that the T-1000 was already there and had assumed the identity of John's foster mother after killing her.
John soon came to realise that everything his real mother had said about the future war was true and decided that he and the Terminator should head to Pescadero Hospital to save her before the T-1000 gets to her. The T-800 advised against it, but was programmed to follow John's orders and so, after John ordered the android not to kill anyone, they set out to find Sarah. They arrived just as Sarah was attempting to escape on her own and stepped in to help her. Initially Sarah was terrified at the sight of a Terminator identical to the one that had hunted her years ago, but soon recognised the real threat that was the T-1000 as it relentlessly chased after them. After fleeing from Pescadero and evading the liquid-metal android, the Connors and their cybernetic protector took shelter at a garage. There, the T-800 revealed that its neural-net processor was set to read-only, meaning it could not learn independently unless its CPU switch was reset. John suggested resetting the switch so that the Terminator could learn how to better interact with people, though Sarah didn't trust the machine and almost destroyed its CPU chip with a hammer. John managed to convince her to spare it, calling the machine a friend rather than just a tool.
After having its CPU reset, the T-800 began to learn how to talk more like a normal person (or at least John's idea of normal speech). It also informed the Connors of the details behind SkyNet's creation and how it triggers Judgment Day, the nuclear event that leads to the near-extinction of the human race. The trio head out of LA to meet up with a Mexican friend of Sarah's - Enrique - who smuggles weapons and taught Sarah and John how to fight. After learning the identity of SkyNet's chief programmer - Miles Benett Dyson - Sarah takes a rifle and a jeep to his house to assassinate him in order to prevent the war. She almost succeeds, but after seeing Dyson is a family man she finds herself unable to pull the trigger. The Terminator and John arrive just in time to stop her and the T-800 reveals its true nature to Miles, going on to explain the details of Judgment Day and the threat that SkyNet becomes in the future.
After convincing Dyson to help them, the Connors invade the CyberDyne Corporation building in LA in order to destroy all of Dyson's research material. They also steal the severed arm and damaged CPU chip of the first Terminator that Sarah destroyed ten years ago. The Connors escape the building after the T-800 opens fire on the police and SWAT teams, without killing any of the officers. The building is then blown up by Dyson, who was fatally wounded after being shot by the police. Just as the T-800 and the Connors flee CyberDyne, the T-1000 reappears and pursues them all the way to a steel mill. A tanker truck that the T-1000 commandeered is overturned, spilling its liquid nitrogen cargo that freezes the android. The T-800 shatters the frozen T-1000 with a gunshot, but the android reconstructs itself after its fragments are melted down to liquid again by the heat from inside the mill.
The T-800 and T-1000 battle it out inside the steel mill, with the T-800 being severely damaged and briefly shut down after it is impaled by a sharp-ended pipe, damaging its power cell. When its back-up cell kicks in, the T-800 finds the T-1000 cornered above a pool of molten steel and fires a grenade launcher round into its chest. The explosion warps the android's liquid alloy body and sends it plummeting into the super-hot steel, where it meets its end.
With the T-1000 destroyed, the T-800 throws the arm and CPU of the first Terminator into the steel pool then states that he must be destroyed as well otherwise he could threaten the future as Dyson's research had. After a tearful farewell, the Terminator - incapable of self-termination - is lowered on a crane hook into the steel pool by the Connors. At it sinks below the surface, the Terminator's hand raises its thumb in farewell to John. It then submerges completely and melts away.