The plot for Terminator Genisys is discussed in its entirety below.
So, not very many of you saw Terminator Genisys this weekend if the box office stats are to be believed. But if you had seen it, you probably would've left the theater scratching your head. Let's see if I can, briefly, summarize the plot:
The film opens as if T2, T3, and Salvation never happened: Judgment Day is still August 29, 1997 and we open in a future in which it has already occurred. John Connor and Kyle Reese are attacking the site of the time displacement device; after a T-800 goes through to 1984, Connor sends Reese through to stop it, just as we imagine happened in the original film. Unlike the original film, however, something goes wrong: a robot of some kind grabs John and whispers something to the effect of "You didn't think it would be that easy, did you?" into his ear as Kyle goes back in time. Whatever that Terminator did changed the past: When Reese shows up in 1984, he is met by a liquid metal T-1000. The T-800 from the first film, meanwhile, is met at the observatory from the first film by a 20-something Sarah Connor and an aged T-800 named "Pop," who was sent back in time to 1974 to save Sarah Connor from a T-1000 meant to kill her. They kill the T-800 and then save Kyle from the T-1000.
After this, Sarah and Kyle travel through time to 2017—apparently, Sarah and Pop were able to build a time displacement device of their own using parts available in 1984—where they plan on stopping a program called "Genisys" from uploading into every computer system. "Genisys" is actually Skynet, naturally, and it appears to have been built by John Connor, who traveled back in time to 2017 to both build the device and stop Sarah and Kyle from destroying it. John was infected by, oh let's say nanotechnology, and is now a Terminator himself. The movie culminates with John being killed and Skynet being destroyed—unless you stick around for the post-credits scene, where we see that Skynet has actually survived.
Okay, got it? Lotta different things going on there. There are two things I'd like you to consider:
Thing the first: We have no idea who sent "Pop" back in time to 1974—loaded with all the knowledge of what would happen in the first Terminator film—to save Sarah. There's a line in the film explaining that whoever sent him "erased" those files.
Thing the second, and more important: We have no idea who sent the T-1000 back to 1974 to kill Sarah Connor. None at all. I guess we're supposed to assume that it was Skynet? But if Skynet had merged with John Connor—who is very reluctant to kill Sarah when they meet in 2017, at least at first—wouldn't that cause all sorts of problems?
The answer to that question is "no," it turns out. But you wouldn't have any idea from watching the final product up on the screen. You'd have to read this interview with the movie's writers, in which it is explained what happens to John when the Terminator grabs him during the film's opening moments:
What is the moment that changes this new timeline? What’s the thing that got altered to prevent this from being just a remake, essentially?
Laeta Kalogridis: Skynet. You see in the beginning. He grabs John. He’s not from this timeline. He’s from an alternate universe, in the multiverse, another of the many universes that exist. That Skynet is not from that timeline.
Patrick Lussier: It is the understanding that for Skynet, finally realizing that "I cannot just wipe out the humans, I can never defeat the humans unless I have the best weapon that humans have, and that is him."
Laeta Kalogridis: Or, more simply put, if you have a Skynet that has witnessed multiple iterations of the rise of the machines, which Skynet has…
Patrick Lussier: And being wiped out over and over…
Laeta Kalogridis: This Skynet has been to this universe, and this universe, and this universe. That’s why he says, "I came a very long way to stop you." He’s not from here. So he’s watched it. He’s watched it happen a bunch of different times, and each time he’s seen it there is a different result but the same result.
So Skynet [Matt Smith] can now hop between dimensions?
Laeta Kalogridis: This particular Skynet, from another place. This Skynet – not from the original two movies – can.
Now, if you've seen Terminator Genisys, you might be saying to yourself "What in the ever-living-f—k is this?" And you'd have good reason to! Because this is literally never mentioned, not once, in the movie. The whole crux of the plot—entire axis on which it turns—is never even mentioned, in passing, by anyone on screen.
In other words, the geniuses in Hollywood spent, oh, $150 million-plus-marketing to reboot a beloved franchise and then didn't bother telling people how the reboot functioned. There's nothing, no line of dialogue, no passing mention. With some franchises, that might not be a big deal. But considering that the entire theme of the first three films revolves around an extended debate about fate and free will, they might have thrown fans a bone by saying "well, lol, nothing from those films really matters because there are infinite timelines."