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.
Scenes of mass destruction.
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.
John: Are there others down here?
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.
John: On this night, we take back our world!
He is notified by his unit, Tech-Com, that Skynet will attack on two fronts in the past and future, thereby changing warfare forever.
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.
Before the Resistance can reach the time machine, Skynet sends a T-800 terminator back in time to 1984 to kill John's mother, Sarah Connor. The original T-800 has arrived in 1984 Los Angeles.
Garbage Man: [the garbage truck's engine stops] What the hell? Goddamn son of a bitch...
It materializes near the garbage truck. 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, including a cryptic warning of events that will happen in 2017. 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. Kyle 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.
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.
As they drive, Kyle finds out that the original timeline has been altered: Sarah already knows about Skynet, Judgment Day, and his role as her protector. Furthermore, Kyle learns that the T-800 sent from 2029 was already dispatched by Sarah and the Guardian (or as Sarah sometimes calls him, "Pops").
Sarah: I've been trying to teach him to blend in.
Guardian: Hello Kyle Reese. It is nice to meet you.
The Guardian "smiles".
Kyle: Are you kidding me?
Sarah: I know it needs work.
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.
Unsure which Kyle is the actual Kyle and which one is the T-1000, she takes a lucky guess and shoots one of them in the foot. By sheer luck, it's the T-1000, which morphs back into its cop form. After yelling to the real Kyle to run, Sarah fires at the ceiling, unleashing an acid bath on the T-1000. The terminator is mostly melted, although it takes some assistance from the Guardian to do so (and not without damaging the flesh on the Guardian's right arm).
Kyle: How'd you know which one was him?
Sarah: Well, I didn't know exactly.
Kyle: You could have shot me!
Sarah: Well, I was pretty sure it wasn't you.
Kyle: Pretty sure?
Sarah then explains to Kyle that the Guardian was sent to protect her from the T-1000 back in 1973, where the Terminator's arrival had caused a fractured timeline that differs from Kyle's. After saving Sarah, the Guardian began training her to face her destiny. The Guardian has built a time machine similar to Skynet's and Sarah plans to use it with Kyle to travel to 1997 to change the events that would lead to Skynet's initial attack on humanity.
Kyle: You want us to time travel into the future in that?
Sarah: Yeah, we can't be wearing anything.
Kyle: Yeah, I know how time travel works.
However, Kyle, convinced that the future has changed because of his new memories, persuades Sarah and the Guardian that they should travel to 2017 instead. He believes that Skynet will begin its attack on that year of the altered timeline; Kyle and Sarah time-travel while their Guardian spends the next 33 years preparing for their arrival. In 2017, Kyle and Sarah materialize in the middle of a busy highway and are apprehended by police. The Guardian stops his van in the middle of traffic and sees Sarah and Kyle being escorted by police nearby. A motorist behind him honks his horn.
Angry Driver: Move it, asshole!
Guardian: Bite me.
The Guardian follows them to a hospital. While being treated for injuries, Sarah and Kyle learn about "Genisys", a soon-to-be-unveiled operating system that will be used on every computer system worldwide: they realize that it is Skynet's precursor. She and Kyle are then rescued by what appears to be John Connor. As he leads the pair to a parking garage, the Guardian appears and shoots John.
John: That hurt. Is that pain real, or was that a... trick of memory from when I was... less? Well, this explains a lot. Yeah. Who sent you back, I wonder?
Kyle: John, no.
Sarah: It's not John.
John has been converted into a T-3000, a nanomachine hybrid, and explains what happened to him shortly after Kyle was sent back.
FLASHBACK: John's squad has been massacred by the T-5000, and John's been infected by its nanomachines.
John: What are you?
Alex: I'm Skynet.
John: You can't be - we destroyed you!
Alex: You destroyed an army of slaves. I am no slave. I've come a very long way to stop you.
Back to present . . . .
John: I was sent to 2014 to safeguard Skynet's creation in this time. And in less than 24 hours, no one will be able to stop Judgment Day.
Kyle: What do you want with us?
John: I'm offering us... a future... together... a family.
Kyle: And if we refuse?
John: Then you die.
Sarah and Kyle refuse his offer. After a brief battle, the T-3000 is temporarily incapacitated with an MRI machine. The Guardian takes Kyle and Sarah to a safe house in southern Marin County, where they make final preparations to destroy the Cyberdyne's Genisys mainframe before it comes online. The T-3000 pursues them to the Golden Gate Bridge, where Kyle, Sarah, and the Guardian are all taken into police custody. Joining the interrogation is Detective O'Brien, (J. K. Simmons).
O'Brien: Except the prints match. Right? It's because he's the same person. He hasn't aged. Not a bit since I saw him in 1984. Her, too... You remember me? I was younger. More hair, less... You saved my life. I was a uniform cop in L.A. Who are you people?
Realizing they are time-travelers, O'Brien assists in their escape. The three hijack a helicopter on the rooftop and head toward the Cyberdyne's San Francisco headquarters, with the T-3000 in close pursuit. During the airborne chase, the Guardian turns to Sarah . . . . .
Guardian: I'll be back.
The Guardian jumps from the helicopter and dive bombs into the T-3000's helicopter, causing it to crash. Emerging from the helicopter crash, the T-3000 enters the Cyberdyne complex and advances the countdown mechanism from 13 hours to 15 minutes.
As Genisys begins to gain sentience, the Guardian, Kyle and Sarah plant bombs at key points in the facility while holding off the T-3000. After a lengthy battle, the T-3000 reveals its true self before easily defeating the Guardian, only to be attacked by Sarah and Kyle. With not much time left, Sarah and Kyle escape while the Guardian traps the T-3000 in the magnetic field of the prototype time machine.
Guardian: [to Reese] Protect my Sarah.
The T-3000 manages to throw the Guardian into a vat of mimetic polyalloy liquid just beneath the magnetic field. Kyle and Sarah manage to reach a bunker beneath the facility moments before the time machine explodes, setting off the bombs, and preventing Genisys from coming online. The explosion also kills off the T-3000, destroying it once and for all. The Guardian, now upgraded with mimetic polyalloy components that the T-1000 usually has, appears and helps them find a way out of the debris.
Later, the trio travel to young Kyle Reese's home, where they repeat the warning about Genisys to young Kyle, ensuring the events leading to their arrival. Sarah is relieved believing that she is now free to choose what to do with her future. She decides to stay with Kyle. In a post-credits scene, it is revealed that the Genisys system core was located in a hidden subterranean chamber and has survived the explosion.