The Termination of Skynet was an attempt to stop Skynet from coming online in the form of Genisys in 2017.
Prelude
At an unknown point, Skynet sent a T-1000 back to 1973 to terminate a young Sarah Connor before she could grow up and give birth to Resistance leader John Connor. In response, an unknown party sent the T-800 known as the Guardian back to save her. Sarah was protected from the T-1000 by Guardian and as her parents were killed, raised by him. As a result, when the first T-800 arrived in 1984 to terminate Sarah before she could get pregnant with John, Sarah and Guardian were waiting for it. The two quickly deactivated the T-800 before it could harm anyone and rescued Kyle Reese from the T-1000. Though the T-800 was briefly reactivated by the T-1000, both Terminators were eventually destroyed. While Sarah and Guardian had initially planned to use their crude time machine to travel forward to 1997 to prevent Judgment Day, the timeline was altered by Cyberdyne not having the T-800's CPU and arm to work off of and Skynet was not created at that time. Instead, while time traveling, Kyle had received memories of the altered timeline where he was given a message that "Genisys is Skynet" and that it could be stopped before it was born in 2017. As a result, Kyle and Sarah instead used the time machine to travel forward in time to October, 2017 while Guardian stayed behind to "take the long way" and prepare for their arrival as damage to its skin sheath prevented it from being able to make the trip. During the next thirty-three years, Guardian got a job as a construction worker and before being laid off, helped build the building where Genisys was housed. He purposefully included a bunker underneath that Sarah could access so they could survive the destruction of the building and created a safe-house stocked with weapons.
Unknown to the three, Skynet, in the form of the T-5000 known as Alex, had turned John into a T-3000 loyal to it. To ensure its creation in this new timeline, Skynet sent John back to 2014 where he worked with Cyberdyne Systems to build Genisys, a revolutionary operating system that connected all of the world's hardware, including military systems. In reality, Genisys was the new timeline's form of Skynet and once it came fully online, it would become self-aware and launch Judgment Day. John used his future knowledge to give Genisys advanced programing and create vats of unprogramed mimetic polyalloy for it to use and started building a time machine, though by 2017, only the quantum field generator was complete.
The Battle
Kyle and Sarah arrive in the middle of an overpass in San Francisco where Guardian is supposed to meet them, but got stuck in traffic. As a result of their dramatic arrival, the police think they are terrorists who detonated a bomb of some sort in the middle of the bridge and arrest them and take them to the hospital to be examined. They discover that there's no record of Sarah anywhere and Kyle's fingerprints match a twelve-year old boy who got into trouble and his parents used the police to scare straight, unaware that the boy is the twelve-year old Kyle of that timeline. Detective O'Brian arrives trying to talk to them and get answers as they had saved him from the T-1000 in 1984. However, he's dismissed as crazy as is Kyle's story and the cops decide to turn them over to Homeland Security. As Kyle and Sarah free themselves from their handcuffs, John arrives to help them escape, telling Kyle that he traveled back in time shortly after Kyle did. He also reveals to a shocked Kyle that Kyle is his father. Kyle and Sarah tell John their plan, but are interrupted by Guardian who shoots John, to Kyle and Sarah's horror. Kyle accuses Guardian of being an Infiltrator sent to gain their trust so that he could kill John and a stand-off ensues until Sarah gets Guardian to stand-down. However, John's wounds suddenly heal and he rises with his facial scars disappearing. John explains that when Kyle was sent back in time in the future, Skynet, in the form of the T-5000 Alex, converted him into a T-3000 loyal to it. He was sent back to 2014 to ensure Skynet's survival and asks his parents to join him. They refuse and Guardian and John fight. Eventually, the two get sucked into the magnetic field of an MRI which Kyle shuts down long enough to free Guardian before trapping John again. Despite John's pleas to help him, Kyle cranks up the power and they flee. John eventually manages to escape the magnetic field, but they are gone by then.
Heading to a safe house near the Golden Gate Bridge, Kyle, Sarah and Guardian discuss the situation. Guardian explains that near the end of the war, Skynet had tried to create human-machine hybrids like John, but they had gone insane and died. John is the only one to survive and is thus unique. Guardian also informs them that John has been changed on the cellular level and can't be cured. When asked how to stop him, Guardian suggests that a magnetic device could be used to restrain John. At the safe house, Kyle and Sarah prepare weapons while Guardian prepares a magnetic glove device for restraining John. John suddenly appears, having known of the location from Sarah telling him how she'd visit it all the time growing up with her father. John again offers them to join him, but they refuse and flee, blowing up the safe house along the way.
Outside, they reunite with Guardian and steal a school bus. John steals a motorcycle and drives it on top of the bus and attacks. Kyle and Guardian try to shoot John, but he goes under the bus and pulls Guardian off. Guardian hijacks a police car and follows. Under the bus, John rips out the break line, preventing Sarah from stopping the bus and as they cross the Golden Gate Bridge, makes the bus flip, sending it teetering over the edge of the bridge. As Kyle and Sarah try to climb out, John climbs aboard and resists their bullets. Kyle hits him with Guardian's magnetic glove, stunning him and sending him to the bottom of the bus. The bus falls into San Francisco Bay, taking John with it, but Sarah and Kyle are rescued by Guardian. They are surrounded by police and unable to safely fight their way out, surrender.
The three are taken to the police station where they are separately questioned. As they are, John arrives disguised as a police officer and shoots several others. Hearing the shots, Guardian breaks free while Detective O'Brian, after being told they're trying to save the world, frees Sarah. She sends him after Kyle while she escorts the twelve-year old Kyle and his family to safety as they had been brought to the police station to see if they knew Kyle due to them having the same fingerprints. O'Brian, Kyle, Sarah and Guardian reunite and collect their weapons then head to the roof to steal a helicopter. O'Brian promises to send the cops in the wrong direction, but they come under attack by John and O'Brian is slightly wounded. Kyle, Sarah and Guardian flee in the helicopter for Cyberdyne, but are chased by John in another helicopter. John damages theirs with gunfire while they are unable to stop his. Finally, Kyle gets their helicopter above John's and has Guardian dive-bomb it, sending it crashing into the bay near Cyberdyne. John emerges from the wreckage and heads into Cyberdyne, killing everyone in his way.
Kyle and Sarah land at Cyberdyne and head inside to be confronted by a hologram of Genisys which grows older and the countdown until its activation is sped up by John from thirteen hours to fifteen minutes. At that point, Genisys will spread everywhere and be unstoppable. John confronts them and they are unable to stop him, but Guardian reappears and spears John with a lamp post, disabling him temporarily. The three then head inside to set explosives and destroy Genisys before it can come online.
The three begin setting explosives with an ever-aging Genisys taunting them. Eventually, John frees himself and attacks, capturing Sarah who tosses Guardian the detonator and orders him to blow the place up. However, Guardian is unable to do so as long as Sarah is alive due to its primary directive being to ensure her survival, thus John purposefully doesn't kill her. However, Kyle finds them and Guardian goes to toss him the detonator so he can destroy the building. Seeing this, John attacks Guardian and destroys the detonator. The two Terminators battle while Kyle and Sarah try to reach them, but are blocked by a blast door by Genisys. The two are able to use one of the explosives to blast through it and rush off to help their friend. Meanwhile, Guardian fights John, slashing him several times with lasers and eventually destroying his human disguise, but is severely outmatched and has his left arm ripped off and is tossed in front of the Time Displacement Equipment. Before John can destroy him however, Kyle and Sarah attack him, giving Guardian time to recover his magnetic glove and punch it through John's chest, restraining him. Guardian orders Kyle to activate the Time Displacement Equipment as its magnetic field will destroy anything non-organic and he can't restrain John for long. Kyle reluctantly does so and drags Sarah to Guardian's bunker as Guardian drags John into the Time Displacement Equipment.
Inside the magnetic field, Guardian restrains John as long as he can, but John eventually breaks free and fights to escape the magnetic field. Guardian fights him back repeatedly until John sends him flying out of the field and into the mimetic polyalloy. John himself is unable to escape the field and is torn apart and destroyed by it. The Time Displacement Equipment explodes as a result, setting off Sarah, Kyle and Guardian's charges and destroying Cyberdyne just as Sarah and Kyle manage to get inside the bunker.
Aftermath
Destroying Cyberdyne prevents Genisys from going online and Judgment Day is averted. Sarah and Kyle are trapped in the bunker, though Kyle points out how they stopped Skynet and saved the future so no matter their fate, its still good. Metallic blades, like those of John in his T-3000 form break through the door and pull it free, but it turns out to be Guardian who bonded with the mimetic polyalloy which repaired and upgraded him so he now has abilities similar to a T-1000 With Guardian's help, they get to safety. To ensure things go the way they did, Kyle visits his younger self and gives him the message he remembered getting that led them to Genisys in the first place. Sarah and Kyle decide to make their own future and kiss, apparently starting a romantic relationship with the approval of Guardian.
However, unknown to anyone, Genisys' system core was actually underground and thus survived the destruction of Cyberdyne. Skynet, in the form of Genisys, survived the attempt on its life, though its plans were delayed with the destruction of its facilities.