The machine overlords of the year 2029 are tired of fighting the pesky human resistance and come up with a plan to circumvent history. They send a human looking robotic killing machine (a terminator) back through time to kill the mother of the resistance leader; thereby preventing his birth and saving them a huge hassle. Arnold Schwarzenegger's terminator arrives in the Los Angeles of 1984 and quickly gets to work, murdering a couple of SoCal punks for their clothes (if you blink you'll miss a young Bill Paxton in this scene) and discovering via the phone book all the women in LA who share the name of his target: Sarah Conner. Shortly afterward Michael Biehn's Reese arrives via the same time displacement equipment the terminator used. His mission: to prevent the terminator from killing Sarah Conner. Given that, in Reese's own words, the terminator "can't be bargained with... can't be reasoned with... doesn't feel pity, or remorse, or fear (and) absolutely will not stop, ever, until you are dead!" and is virtually indestructible on top of all that it's fair to say that Reese has his work cut out for him.
The Terminator cuts a swath of destruction across LA in his quest to find and kill his target with Reese and Sarah Conner barely a step ahead of him (Cameron's use of chase scenes as platforms for exposition makes it seem as though the film simply never takes a breath). Sarah's initial reluctance to believe anything Reese is telling her gets blown away when the terminator tracks her down to a police station, drives a car through the front doors (after Ahnuld utters what was to become his signature line; "I'll be back") and proceeds to spray wall to wall death and destruction upon the ineffective cops. After narrowly escaping Reese and Sarah find one night of peace together during which time John is conceived but they're soon tracked down again and the chase resumes. After fleeing back toward the city the terminator chases them down and there's the climactic encounter in a factory.
In my opinion the plot wasn't what made this work so successful as a motion picture, though it is a strong script. So what was? Well, to begin with the film looks cool. Cameron creates a Los Angeles that could be the precursor to the LA in Ridley Scott's "Blade Runner", but more fundamental to the films success is the blissful lack of post-modern irony. Reese is not an anti-hero, he's an old-fashioned hero ready to give his life for the greater good. Schwarznegger's terminator has absolutely no redeeming value or sympathy-evoking root causes for his behavior. He wasn't a good kid who's gone bad. He's death on two feet. He is the void. His job is to kill you, period. In a world increasingly overrun by moral relativity Cameron's simple good vs evil paradigm was refreshing and easy for average movie goers to get their head around. There were no inconvenient pangs of guilt clanging around in the subconscious mitigating your enjoyment of seeing the villian get his comeuppance.
Casting Schwarznegger against type was also a stroke of genius. Ahnuld's agent was against him taking the role, concerned it would ruin his public image, but Cameron knew better. He knew that Schwarznegger's steely tuetonic presence was perfect for the terminator and he was right. Given virtually no lines to screw up Ahnuld was able to make the role his own and his career took off like a rocket after this film. Michael Beihn, Lance Henricksen, Earl Boen and Paul Winfield deliver strong supporting performances with the only weak link in the entire production chain being the in over her head Linda Hamilton. The woman simply can't act. There isn't a moment in the entire film that I believe she is Sarah Conner. When compared to Sigourney Weaver's iconic performance in Cameron's 1986 masterpiece "Aliens" Hamilton's performance here is, frankly, embarrassing (as was her work in T2). That said, the special effects were also first rate for their day and stand as a testament to how well, even at this early point in his career, Cameron understood the craft of film making.
Put all those elements together: compelling concept, easy to understand good vs bad moral structure, career making performances, strong script, deft direction and good special effects and you have a movie worth spending a couple of hours with. The Terminator was not in and of itself a game changer, but it certainly helped push contemporary cinema of the time toward the 21st century.
The Terminator 1984 - trailer