With timelines being altered all over the shop, Sarah and Kyle time travel out of 1984 into 2017, leaving behind 'Pops' (an aging Arnie) to recover his skin, and prepare for their revival.
Even though he has some 33 years to prepare, pops is late to meet them, and they are picked up by cops for exposure and causing carnage on the freeway. At the hospital, John Connor (from the future) finds them and rescues them to everyone's surprise ... until Pops shows up and shoots him. It turns out that John Connor is now Skynet with a new tactic of theirs ("if you can't beat em..."), and on a mission to deliver Judgement Day, again, with the launch of a new tame-looking device-connection app that's going to be bigger than Angry Birds.
Pops and the evil JC fight it out, until they manage to stick the Terminator hybrid to an MRI machine and make a run for it. They go to a hideout Pops has been preparing, but waste precious time loading the guns with ammo, as it turns out he's a lazy old thing, and John shows up.
Trying to shake him off, they make it to the Golden Gate Bridge, but come off second best. Pops rescues his two friends, but the cops show up again and take them in. J.K.Simmons, who was the young officer on duty in 1984, believes their story and breaks them out just in time. They hatch a plan - to get to the HQ of Cyberdyne, and blow it up before launch.
They take a chopper, but John is way ahead of them and has one too. They dance around a little, until pops jumps out, does a little skydiving, then plunges into the other helicopter, buying his friends some time.
Kyle and Sarah make it to the HQ, but already the countdown clock has sped up. A VR hologram tries to stop them as they plant bombs around the place. John is soon there, and Pops is on the scene to save the day. He traps John to face the detonation with him, but the T-3000 finally dispatches with the weaker T-800 model of Arnie and hurls him into a metallic lake.
The bombs, and a time machine in the building, blow up, destroying the T-3000 and stopping Skynet. Kyle and Sarah and trapped - until they are rescued by a suped-up version of Arnie, who is now a T-1000 with the help of the metallic polymer lake he fell into.
Kyle stops by his younger self to give him some advice, before they hit the road.
An end credits scene tells us that the Skynet core is STILL not dead and survived the bomb blast.