Over an apocalyptic battlefield in the year 2029, the ruins of the city of Los Angeles are dominated by robotic war machines, some like tanks, others are hover-crafts, a title card tells us that in the future, humans will wage a long war against an enemy known as the Machines. The "final battle" however, will not be fought in this future world, but in the past, specifically our present... tonight.
The year switches to Los Angeles, May 12, 1984 at 1:34 am. At the Griffith Park Observatory overlooking the city, lightning emanates from above a garbage truck, knocking out power. The driver sees an object ahead of him and flees as the electrical surge dissipates. The object is the Terminator (Arnold Schwarzenegger), a naked and heavily muscular cyborg, and he stands up and starts walking toward the city where he is spotted by three young punks. The Terminator orders the men to give him their clothes, and the punks produce knives in response. The Terminator swats two punks aside, the third stabs him, but the Terminator tears open the punk's body with his bare hands, and kills him. One of the subdued punks immediately takes off his jacket and offers his clothes to the Terminator.
In a downtown alley, a homeless man sees a bright, circular light just above the ground, similar to the one at Griffith Park. A scarred, naked man, muscular but much smaller in size than the other man who arrived in a similar fashion, is forced through the portal and lands in the alley, in obvious pain. This is Kyle Reese (Michael Biehn). Reese takes the homeless man's pants, just as a police car pulls up and two cops yell at him to freeze. Reese is able to hide and attacks one of the cops, taking his pistol and demanding the cop tell him the date and year. The cop becomes puzzled by Reese's question. When the cop's partner arrives, Reese runs into a department store. He steals several items, including a pair of Nike sneakers and a long coat and escapes the store. In another alley outside, he steals a shotgun from an unoccupied squad car. Finding a phone book nearby, he looks up the name "Sarah Connor."
Elsewhere a young woman, Sarah Connor (Linda Hamilton), lives the life of a lonely waitress. Sharing an apartment with her friend Ginger, Sarah is living out a boring life that seems to go nowhere.
The Terminator steals a car and goes to a local sporting goods store where he has the owner (Dick Miller) show him several assault weapons and a pistol with laser sighting. He asks for a "phased-plasma rifle"; the owner tells him "only what you see, pal". The owner tells his customer that he can take the rifles right away but the pistol has a waiting period. As he explains the process, the Terminator casually loads the shotgun and shoots the clerk. Shortly after, the Terminator finds a phone book and looks for the name "Sarah Connor" finding three listings in the Los Angeles area. He goes to the address of the first listing; as he walks to the front door, a small dog barks at him. When this Sarah Connor opens the door, the Terminator forces his way inside and shoots her with the pistol he acquired. Not long after, at the diner where she works, Sarah's co-worker drags her to the television where she sees a news report about a woman sharing her name having been murdered by an unidentified assailant.
That evening, Sarah and her roommate, Ginger, prepare for separate dates. Ginger's boyfriend, Matt, will be coming over to spend the night. Sarah gets a message from her date, who cancels. Sarah decides to go out for pizza and sees another news report where the police announce the death of another woman sharing her name. Sarah becomes worried, and when she sees she is being followed (by Kyle Reese), she ducks into a small dance club called Tech Noir. She tries to call Ginger, however Ginger and Matt do not hear the phone as they are having sex. Not long after Sarah's call, the Terminator attacks and fatally shoots both Ginger and Matt, before hearing Sarah's voice on a message machine saying where she is. Sarah then phones the police department and is connected to Lt. Traxler, the detective investigating the Sarah Connor killings. He tells Sarah to stay put until he can get a squad car to her. The Terminator arrives at the club, dispatches a bouncer, and works his way inside.
In the meantime, Reese has also entered the dance club. Sarah is spotted by the Terminator, which aims its laser sighted pistol at her. Reese fires on the Terminator, hitting it with several blasts and knocking it to the floor. He tells Sarah "Come with me if you wanna live" and Sarah sees the Terminator impossibly rise to its feet. Reese and Sarah escape through the back door of the club, with the killer chasing them -- as he chases them, we learn that the Terminator is a robot of some kind who sees them through a computerized form of red night vision. Reese blasts the gas tank of a nearby car which explodes. The Terminator is unfazed, and jumps on the hood of Reese's car, smashing its fist through the windshield and grasping at Sarah. Reese maneuvers his car and throws the killer off. Reese speeds off while the Terminator subdues a cop (screenwriter William Wisher) and takes his car.
During the chase, Reese frantically identifies himself and explains that Sarah has "been targeted for termination" and that the killer chasing them is not a man but a machine called a "terminator"; a metallic combat chassis covered with living human tissue to make it appear human. (Known as a Cyberdine Model 101 Infiltrator Unit, Type 800 Series in further sequels.) Reese tells Sarah that the first and larger 600 Series Terminators had rubber skin, but the newer 800 Series are very hard to identify which is why he followed Sarah and waited for the Terminator to make its move so he could identify it.
While hiding out in a parking garage, Reese also explains that Sarah is destined to give birth to the humans' future leader John Connor. Reese goes on to say that a nuclear war will be initiated by a new, powerful computer system (referred to as "Skynet" in the first sequel) that is going to be tasked with controlling all defense systems. Reese himself has not seen the ensuing nuclear holocaust but was born and grew up destitute and starving in the ruins. He was enslaved and marked with a barcode and was forced to work loading bodies into incinerators. The human race, he says, will be on the verge of extinction when Sarah's son, John, is able to organize the remaining humans into an effective resistance movement that, by the time Reese will be sent back to the present day by Connor himself, had actually defeated Skynet. In desperation, Skynet has sent the Terminator to the present day to murder Sarah and eliminate John Connor's existence. Reese also explains that the Terminator pursuing Sarah, is a new model, one that appears infinitely more human than its predecessors. Reese tells her the android will bleed, sweat and even has bad breath to enhance the disguise. Reese is uncertain that he can defeat the android without having the advanced weaponry of the future.
While hiding out in a parking garage, Reese gets rid of his first car and steals a second one. The Terminator finds them there and the chase resumes. Reese has Sarah take over driving and is able to hit the Terminator with a few blasts from his shotgun. Sarah stops their car and the Terminator crashes into a wall. When she and Reese are arrested, she sees that the Terminator has escaped the scene.
At Traxler's police precinct, Sarah is told that Ginger is dead and that Reese has been given to a criminal psychiatrist, Dr. Peter Silberman (Earl Boen). His story about the Terminator is treated as the babbling of extreme delusion. While watching the videotape of the interview, Silberman eagerly says that a case like Reese's could be career-making. During the interview, Silberman asks Reese why he didn't bring weaponry from the future with him, to which Reese replies that only living material will go through the time portal. Reese quickly becomes agitated and begins to scream into the camera that the Terminator must be destroyed or it will not stop until it kills Sarah.
The Terminator goes back to a shoddy hotel room where it hides out. It removes one of its eyes that had been damaged when it was shot by Reese. Under the eye is a sophisticated robotic eye that glows red. It dons a set of sunglasses, changes into different clothes and marches out to resume the hunt for Sarah, taking a pump-action SPAS shotgun and an AR-15 automatic rifle.
At the police precinct, the Terminator arrives in the foyer just as Dr. Silberman is leaving and asks to see Sarah. The desk sergeant refuses to let the Terminator in. The Terminator looks menacingly at the desk sergeant and simply states "I'll be back." The Terminator goes outside and one minute later crashes a car through the front door. It marches through the precinct, ruthlessly shooting every officer in its path. The Terminator gets to a circuit panel and after ripping out a main circuit cable, shoves the live wire into the fuse box creating an electrical surge that blows out all lights in the building, leaving the policemen at a disadvantage. In the battle, Traxler is killed and Reese escapes confinement. He finds Sarah and the two escape the precinct.
While hiding out in a sewer tunnel, Sarah realizes that Kyle's story is true. Reese tells her more about the future where humans barely survive amid the wreckage of cities and the predation of the Terminators. Initially, Skynet's vehicles, "hunter-killers" or HKs, would use infra-red technology to find and kill humans. Kyle mentions that John Connor's talent for strategy helped the Resistance to defeat them. Later, the new Terminators, "infiltrators" like the one he and Sarah are on the run from, appeared and began to find hidden bunkers where humans hid out. Sarah lapses into sleep and has a nightmare of a firefight where Terminators break into a human sanctuary and massacre scores of civilians. Kyle himself is there; after finding a place to rest after a Resistance patrol, he takes out a Polaroid photo of Sarah and admires it. Alerted by dogs (who can sense a Terminator) at the entrance to the bunker, Reese joins the valiant fight to destroy a Terminator that has gotten in. The Terminator, carrying a heavy plasma gun, slaughters everyone and causes an explosion that cripples Reese. Reese sees the picture of Sarah being burned nearby.
Meanwhile, the Terminator is back in its seedy motel room hideout looking at an address book that it took from Sarah and Ginger's apartment and sees a list of names including that of Sarah's mother who lives upstate. After harshly dismissing the hotel owner, the Terminator then leaves its room and gets on a stolen motorcycle and takes off on the road.
Later, the two reach a roadside motel, where Kyle goes to purchase chemicals needed to make explosives. While he is shopping (using "borrowed" money) Sarah showers and phones her mother, telling her the phone number where she is, unaware that the Terminator is on the other line and recreating the voice of Sarah's mother. Sarah's mother is nowhere in sight, but the appearance of the cabin's knocked down front door with a large shotgun blast hole in it suggests that the Terminator forced its way in and probably shot Sarah's mother and waited for Sarah to call. The Terminator then phones the motel and, using its real voice, asks the desk clerk to give him the motel's address.
At the motel, Sarah and Kyle make a stockpile of pipe bombs filled with plastique, a compound that Kyle had learned to make and teaches Sarah to make as well, also showing her the cautious process involved to make the bombs. When Sarah asks Kyle if he has ever had a lover, he replies he has not, a fact that touches Sarah. Kyle then mentions having a photo of Sarah and how he has fallen in love with her. Realizing he has gone too far, he furiously begins loading explosives, but Sarah stops him and kisses him. The two lose their inhibitions and have sex. Their consummation results in the conception of Sarah's (and Kyle's) son, John.
That evening, the Terminator tracks them to the motel and the two flee again in a stolen pickup truck (alerted by a tenant's pet dog barking at the sight of the approaching Terminator). As they race down a wide highway, Reese makes a valiant effort to destroy the Terminator with the pipe bombs he and Sarah made, however, he is hit by gunfire. Sarah manages to knock the Terminator off its motorcycle and her truck flips over.
The Terminator recovers and is immediately struck by a semi-tanker truck and dragged for a short distance. After the driver stops, the Terminator kills him and takes control of the truck, attempting to run down Sarah. Sarah is able to get Kyle out of their wrecked pickup before the Terminator runs it down.
Near a factory, Kyle uses another pipe bomb to detonate the truck's tank trailer, which explodes. As Reese and Sarah embrace, the Terminator emerges from the wreckage, its flesh completely burned off. Reese and Sarah retreat into the factory, which is automated. Reese switches on as many of the machines as he can, making tracking more difficult for the Terminator. They are finally cornered and Reese places his last bomb in the endoskeleton of the Terminator, which explodes, scattering pieces of the android. Sarah, nursing a severely injured leg, finds Reese, but he's dead.
She is suddenly attacked by the top half of the Terminator's skeleton. She desperately crawls away, finally luring the Terminator into a giant hydraulic press. She traps it there and, as it mindlessly tries to break her neck, pushes the button activating the press. The Terminator is crushed until its red glowing eye fades, signalling its death.
Sarah is taken to an ambulance and sees the paramedics loading Kyle into a body bag. Later, she is driving a Jeep in the desert, a large dog in the passenger seat, seemingly towards Mexico, stopping at a gas station. She is more visibly pregnant with John Connor and has been recording her voice using a cassette player; the tapes are for her son. One of the questions she poses is whether or not she should tell John about Reese being his father and if that will affect his decision to send the warrior back in time to meet and save Sarah. While the Jeep's tank is filled, a young boy takes her picture with a Polaroid camera (it is the same picture that John will give Reese in the future). She and the boy bargain over the price and she buys it. The boy says something in Spanish and the gas station owner tells her he said "A storm's coming!" Sarah sees the storm approaching and says "I know." She drives off into an unknown future.