Kyle: [narrating] Before they died, my parents told me stories about how the world once was; what it was like long before I was born; before the war with the machines. They remembered a green world, vast and beautiful, filled with laughter and hope for the future. It's a world I never knew. By the time I was born, all this was gone. "Skynet," a computer program designed to automate missile defense. It was supposed to protect us, but that's not what happened. August 29th, 1997, Skynet woke up. It decided all of humanity was a threat to its existence.
Kyle: It used our own bombs against us. Three billion people died of nuclear fire. Survivors called it Judgment Day. People lived like rats in shadows, hiding, starving, or worse, captured and put into camps for extermination. I was born after Judgment Day, into a broken world ruled by the machines. The worst were infiltration units that posed as humans. We called them Terminators. And then one man found me. His name was John Connor, and he changed everything. In a subterranean tunnel, John rescues the young Kyle from an approaching terminator.
Kyle: John showed us how to fight back; how to rise up. He freed prisoners. He taught us how to slash the machines to scrap. People whisper about John and wonder how he can know the things he does. They use words like prophet. But John's more. We're here because tonight, he's going to lead us to crush Skynet for good.
In the year 2029, John Connor (Jason Clarke), leader of the human Resistance against Skynet launches a massive final offensive on Los Angeles. He is notified by his unit, Tech-Com, that Skynet will attack on two fronts in the past and future, thereby changing warfare forever. While John gives his speech to the members of the Resistance, we see his plan in action.
John: If we die tonight, mankind dies with us. I look at each of you and I see the marks of this long and terrible war. On this night, we take back our world!
Connor sets up two attack forces, one to strike at Skynet's main defense grid in Colorado, and a second one, led by John and his trusted lieutenant Kyle Reese (Jai Courtney), to destroy Skynet's main weapon, a time machine hidden at a remote storage facility.
As John and Kyle battle their way to the storage facility, Skynet is already deploying a T-800 terminator back in time to 1984 to kill John's mother, Sarah Connor. At that moment, they receive the news that the Colorado mission is a success - Skynet is destroyed. John and Kyle watch all the terminator and aircraft battle units deactivate. But it's too late.
The original T-800 arrives in 1984 Los Angeles.
Garbage Man: [the garbage truck's engine stops] What the hell?
The T-800 scans the horizon then steps forward to look over the city.
Reese volunteers to go back in time in order to protect Sarah and ensure John's existence. Moments before Kyle disappears, he witnesses a cyborg posing as a Resistance soldier suddenly attack John.
Alex: [to John] You didn't think it would be that easy.
As Reese floats in the machine's magnetic field, he receives fragments of memories belonging to an alternate 12-year-old version of himself (Bryant Prince) with a young Sarah, and receiving a Genesis unit as a gift. Young Kyle recites to himself in a mirror . . .
Young Kyle: Remember, Genisys is Skynet. When Genisys comes online, Judgment Day begins. You can kill Skynet before it's born.
Reese materializes in a back alleyway, in 1984 Los Angeles. He steals a pair of pants from a homeless man.
The naked T-800 goes to the Griffith Park Observatory where it confronts three punks.
Punk #1: Nice night for a walk, eh?
Terminator: Nice night for a walk.
Punk #2: Wash day tomorrow. Nothing clean, right?
Terminator: Nothing clean, right.
Punk #1: Hey, I think this guy's a couple cans short of a six-pack.
Terminator: Your clothes, give them to me. Now.
Punk #1: [pulls out a knife] Hey, fuck you, asshole.
All seems to go just like it happened in the first film, that is, until a new, much more aged T-800 (Arnold Schwarzenegger), later in the story known as Guardian, appears to confront the original.
Guardian: [from behind] You won't be needing any clothes.
Young Terminator turns around to find the Guardian approaching him with a shotgun. The older Terminator unhoods himself.
Guardian: I've been waiting for you.
The Guardian shoots the young Terminator, but it only slows it down. The three punks run away just as the two cyborgs engage in a one-on-one brawl, with the younger one gaining the upper hand. However, just as the younger T-800 is about to finish the older one off, a sniper allied with the T-800 shoots it from the observatory's roof, disabling its onboard computer.
Back at the alleyway, a police car arrives and its officer chases Reese. Reese corners the officer, taking his revolver, and demanding to know the date and year, just like before in the first film. Much to Kyle's astonishment, the cop reveals that he knows Reese was to arrive at that precise time. Reese fires several shots at the officer, revealing that it is a T-1000 terminator. The shots stall the T-1000 long enough for Reese to escape into a nearby department store, where he grabs some new clothing.
While fleeing, he inadvertently runs into two officers, Garber and O'Brien, who subdue and handcuff him. Garber and O'Brien lead Kyle away in handcuffs, failing to see the T-1000 emerge from a dressing mirror. It impales Officer Garber through the back. Kyle and O'Brien manage to take cover behind another counter.
Young O'Brien: That's got to be an alien, like from outer space.
Kyle: It's a machine that kills humans. Uncuff me!
Young O'Brien: No! You're under arrest!
Kyle: What part of "kills humans" is confusing to you?
Just when it seems like their luck is about to run out, an armored van driven by Sarah Connor (Emilia Clarke) bursts through the storefront, and knocks the T-1000 out of the way.
Sarah: Come with me if you want to live! NOW, SOLDIER!
She then empties a Desert Eagle pistol into it, stalling it long enough for Reese to get into the truck with her, and then drives off. As they drive, Kyle finds out that the original timeline has been altered: Sarah already knows about Skynet and Judgment Day.
Kyle: This is all wrong! John sent me here to save you!
Sarah: From the Terminator that was sent back to kill me, I know. But we already took care of him.
Kyle sees the disabled Terminator, then panics when he sees the Guardian, unaware that he's the good Terminator. Kyle takes out a gun with the intent on killing the Terminator.
Sarah: Those bullets will kill him!
Kyle: He's here to kill you. Stay in the truck.
Reese discovers the Guardian is gone. He knocks Kyle unconscious, which Sarah is most displeased.
Sarah: Oh, great. That's just great.
Guardian: I did not kill him.
Sarah: Bite me.
Guardian: That is a very immature response.
Sarah: [looking at an unconscious Kyle] So you're the one I've been waiting for all my life.
Guardian: How is he?
Sarah: Still breathing.
Guardian: Good. . . Then you should be able to mate with Kyle Reese in this timeline.
Sarah: OK. . . We're not having this conversation again. . . Look, it's not just mating. I'm supposed to fall in love with him.
Sometime later, Kyle wakes up and sees the Guardian (or as Sarah sometimes calls him, "Pops") driving the truck.
Guardian: Hello, Kyle Reese. It is nice to meet you.
The Guardian "smiles".
Kyle: Are you kidding me?
Sarah: I've been trying to teach him to blend in. I know it needs work.
Kyle: It looks old. I've never seen a terminator old before.
Terminator: I'm old, not obsolete.
As Kyle is taking in the new revelations, the T-1000 rams into their vehicle and starts shooting.
Kyle: Get down!
Guardian: We've been re-acquired.
Sarah: Uh, yeah, no shit!
The Guardian and Sarah plan to lure the T-1000 into a trap from an abandoned factory. From there, the T-1000 disables the Guardian and re-activates the fallen T-800. The T-1000 then pursues Sarah while Kyle fends off the T-800. After destroying its flesh, Kyle finally destroys the T-800 by blasting its head off.
Kyle: And I volunteered for this shit.
Moments later, Sarah finds herself in a chamber beneath an acid bath set up as a trap for the T-1000, when she's approached by two Kyles.