On May 12, 1984, two beings from the year 2029 arrive in Los Angeles: one is a Terminator T-800 Model 101 (Arnold Schwarzenegger), a cyborg assassin programmed to kill a woman named Sarah Connor (Linda Hamilton); the other is Kyle Reese (Michael Biehn), a human resistance fighter sent to protect her.
After the Terminator kills two other women named "Sarah Connor" listed in the telephone directory, it eventually tracks down its target to a nightclub. Kyle shows up and saves Sarah from the Terminator's attack and the two make an escape.
Kyle explains to Sarah that in the near future, an artificial intelligence defense network called Skynet will become self-aware and initiate a nuclear holocaust of mankind, also explaining that he himself was born after it. Sarah's yet-to-be-conceived son John will rally the survivors and lead a resistance movement against Skynet and its army of machines.
With Resistance on the verge of victory, Skynet has sent a Terminator back in time to kill Sarah before John is born, as a last-ditch effort to avert the formation of the Resistance. The Terminator is an efficient killing machine with a powerful metal endoskeleton, but with an external layer of living tissue that makes it resemble a human being.
Kyle and Sarah are again attacked by the Terminator, leading to a car chase. Their cars eventually crash and break down. Kyle and Sarah are caught by the police, but the Terminator disappears. Kyle is questioned by criminal psychologist Dr. Silberman (Earl Boen) who concludes that he is paranoid and delusional.
Sarah is also questioned by Lieutenant Traxler (Paul Winfield) and Sergeant Vukovich (Lance Henriksen) about the events that happened. The Terminator shows up and attacks the police station, killing many police officers including Traxler and Vukovich in its attempt to locate Sarah; however, Sarah and Kyle manage to escape and seek refuge in a motel.
Kyle confesses that he has long been in love with Sarah, having been given a photograph of her by her son John so that he could find her. Sarah reciprocates Kyle's feelings and they have sex that night.
The next day, the Terminator tracks them to the motel and Kyle and Sarah escape in a pickup truck. In the ensuing chase, Kyle throws pipe bombs at the Terminator in an effort to destroy it. Kyle is wounded by the Terminator's gunfire; Sarah manages to knock the Terminator off its motorbike but subsequently loses control of the pickup truck and it flips over.
As the Terminator gets up, it is struck by a gasoline tank truck that drags it a short distance. The Terminator hijacks the truck and resumes the chase. Kyle slides a pipe bomb on to the truck's fuel trailer causing a massive explosion. The Terminator emerges from the burning truck and collapses. Kyle and Sarah embrace one another.
The Terminator, with its flesh coating burned away, unexpectedly rises up and continues to pursue them into a factory. Kyle faces the Terminator by hitting it several times with a metal pipe but it knocks him down. Though dazed, he jams his final pipe bomb into its abdomen, causing an explosion which severely damages it; the explosion kills Kyle and injures Sarah.
Still partially functional, the Terminator resumes its attempt to kill Sarah. She leads it into a hydraulic press and crushes it, causing it to deactivate. Sarah is later taken out of the factory by an ambulance as Kyle's dead body is taken away.
Several months later, a pregnant Sarah is traveling through Mexico. Along the way she records audio tapes which she intends to pass on to her unborn son, John. She debates whether to tell him that Kyle is his father. While stopping at a gas station, a boy takes a photograph of her which she purchases—the same photograph that John will later give Kyle. Sarah then drives on toward approaching storm clouds.