In 1984 Los Angeles, a T-800 Terminator robot and human soldier Kyle Reese arrive nude in flashes of blue lightning. After getting some clothes (the T-800 mercilessly killing some nearby punks while Kyle robs a store), they both look through phone books for their main objective, discovering there are three women in town named Sarah Connor.
One of those Sarahs is an unassuming waitress whose biggest concern is finding a date to go out with her roommate Ginger. After the latest guy bails on her, she heads to a bar alone and is terrified to see on the news that two women with her same name have been murdered. Soon afterward she spots Kyle following her and ducks into a nightclub where she calls the police, who advise her to stay there for now.
The T-800 arrives at Sarahs apartment and kills Ginger and her boyfriend, before Sarah calls and alerts it that it had the wrong woman. She reveals her location, and it goes to the nightclub and is about to kill her when Kyle puts it down with multiple shotgun blasts. But this only momentarily stuns it, and Kyle urges Sarah Come with me if you want to live. The T-800 shoots up the club as they barely escape, and Kyle steals a car and evades the arriving police.
The two lay low at a parking garage where Kyle explains whats going on: a few years from now, the computer intelligence Skynet will take over the world and try to exterminate humanity. Kyle grew up as a slave until a leader emerged to go to war against the machines, which they ultimately won only for Skynet to send the T-800 back in time to kill Sarah. That leader is her yet-unborn son John. Kyle was sent to protect her, though he doubts his chances with 1980s weaponry. The T-800 finds them again using police reports, but Kyle tricks it into crashing its car right into a wall, badly damaging its arm and eye. As the police arrive, it retreats to repair itself while Kyle is arrested.
The police try to come up with explanations for the T-800s apparent invulnerability and tell Sarah that Kyle is simply crazy, but hes proven right when the repaired T-800 returns and rampages through the station, mowing down every cop in his way. Kyle escapes his guard in the confusion and reunites with Sarah, and they escape again. Kyle is shot in the arm, and they share a tender moment as Sarah patches it up and he praises her work.
Kyle and Sarah check into a motel where they set about making pipe bombs and getting other weapons. Kyle reveals that John gave him a picture of her years ago, and he found himself captivated by her sad expression and wondered what he was thinking. He finally admits that he fell in love with her through that picture, and Sarah is so moved that she kisses him, leading to them having sex.
Sarah calls her mother, but is actually talking to the T-800 who had already killed her and is copying her voice. After getting their new location, it arrives at the motel, but a barking dog outside gives enough warning that Kyle and Sarah are able to escape out the back, leading to another chase down the highway. Their bombs eventually crash the T-800s motorcycle, so it breaks into a tanker truck to run them down. Kyle blows the truck up with another bomb, and it momentarily seems to be over, only for the T-800 to still stand back up after its human disguise melts off.
Kyle and Sarah run into a manufacturing plant, where Kyle pushes Sarah to safety as he fights the T-800 one on one, finally sacrificing his life to blow it up with another bomb. However, this still doesnt end the chase as it still crawls after Sarah with its one remaining limb. She lures it into a hydraulic press and crushes it, finally putting an end to it.
A few months later, Sarah records a tape for her unborn son John, revealing that she became pregnant from that night with Kyle. She wonders just how much she should tell John about his true relationship to his friend and father, when a boy takes her picture, revealed to be the same one John will later give to Kyle to start the events which will lead to his birth. She drives off into an approaching storm, facing the bleak future she knows is coming.