The Terminator arrives in 2004 in the middle of the night, just off of a desert road, and walks into a nearby gay bar. As with previous time travellers, the cyborg arrives naked, which excites the bar's patrons. The Terminator acquires clothing from the stage performer - by force, of course - then steals a Land Rover containing a shotgun in the back. It then heads for Los Angeles to carry out its mission: to find John Connor and Kate Brewster (who becomes John's wife and second-in-command in the future) and ensure that they survive Judgment Day.
The Terminator locates Kate Brewster just outside the vetinary clinic where she works and after asking her where John is, locks her in the back of her own van. He then enters the clinic where he finds John fleeing for his life from the T-X: SkyNet's most advanced Terminator model yet that has been sent back to kill him and as many of his future lieutenants as possible. The T-850 took John to Kate's van and told him to flee while he held the T-X at bay. The T-X proved to be exceptionally strong and disabled the less-advanced cyborg with a shot from her plasma cannon.
The T-850 rebooted minutes after this attack and hijacked a police motorcycle to pursue the T-X, which had hijacked a fire engine and taken control of several other vehicles to go after John and Kate. After a lengthy chase, the android catches up to its protectees and they lose the T-X. The T-850 then explains to John that both he and Kate need to survive, that Kate will be in the Resistance, and admits that it is an obsolete design in comparison to the T-X.
The trio later arrive at a cemetery (after the T-850 robs a gas station for food for John and Kate) and the cyborg shows John the grave of his mother Sarah Connor. The T-850 breaks into the mausoleum and opens Sarah's coffin, revealing no body but a stash of weapons. Unable to take in everything that's happening, Kate tries to run away, but the T-850 and John catch up to her after a brief shootout with the police. Kate is narrowly rescued from the T-X, which disguised herself as Kate's fiancee to get close to her.
After escaping from the cemetery and losing the T-X and police, John asks about the T-X's kill list and the T-850 mentions the name of Kate's father, General Robert Brewster. He goes on to explain that Brewster is in charge of Cyber Research Systems, a military technologies group which are responsible for creating SkyNet after CyberDyne Systems was shut down. John orders the Terminator to help them save General Brewster and shut down SkyNet, but the T-850 states that there is no time as SkyNet will initiate its nuclear assault in a matter of hours. Kate begs the Terminator to save her father, and the T-850 complies, revealing that he was programmed to follow her orders and that she was the one who had him reprogrammed and sent through the time displacement field. John asks why he didn't send the Terminator back and it is revealed that this Terminator - shortly before being captured - had terminated John's future self.
The group travel by RV to CRS headquarters and arrive just in time to watch the T-X shoot General Brewster. The T-850 blasts her with a grenade launcher and sends her falling down an elevator shaft, but knows that she'll be back. Moments later, several robots go on the rampage inside the building and start slaughtering everyone in their path. The wounded Brewster tells John and Kate to get him to his office and there they find a file mentioning Crystal Peak, a military bunker that John misinterprets as being the location of SkyNet's system core. Brewster tells John to take care of Kate before succumbing to his wounds.
As the group head for the hangar, the T-X reappears and the T-850 keeps her busy while John and Kate make a run for it. The two androids fight, causing a great deal of damage to their surroundings, but it is obvious that the technologically superior T-X has the advantage. She manages to disable the T-850 and uses her nano-transjector to take control of him. After pulling itself together, the Terminator follows John and Kate down to the hangar as they are about to leave and attacks John. Its CPU is intact but it has no control over its body, and John tries to remind the T-850 of its mission, telling it that killing him will mean he will fail. The T-850 then throws John aside and smashes into a car as it tries to resist the T-X's corruption. After a few seconds of pounding away on the car, it shuts itself down, giving John and Kate the chance to escape to Crystal Peak.
After a few minutes, the T-850 reboots and has apparently purged its systems of the T-X's influence. It takes a helicopter to Crystal Peak where it finds that the T-X has John and Kate cornered outside the slowly-opening blast door. The T-850 intentionally crashes its chopper on top of the T-X, disabling her briefly, and then emerges from the craft with its clothes shredded and half of the flesh on its body gone. The blast door start closing and the T-850 throws itself under the door to keep it open. John and Kate crawl under the door, but the T-X - which has now lost its legs and shed its human disguise - crawls after John and grabs his ankle, breaking it. The T-850 grabs the T-X and holds her while John and Kate disappear into the bunker. The T-850 then overloads its remaining power cell, ejects it and stuffs it into the T-X's mouth. A moment later, the cell detonates and destroys both machines.