The Terminator franchise started with a low-budget thriller film that introduced a very interesting and compelling story about a woman who is a target of a ruthless man from the future that turns up to be a cyborg. It was a simple yet good story and a good piece of science fiction. Why was it a good film? Well, for starters, the bad guy, i.e the terminator, according to Kyle Reese, doesn't feel pain or pity, can't be reasoned with, can't be bargained with. It was the perfect killer, and almost invincible. So there you got all the elements of a good story.
Then came Terminator 2: Judgement day. Here things start to get weird with the new T-1000, because it was the result of CGI of that time. The whys and hows of the T-1000 were cleverly hidden away and never discussed. The story was good: here Sarah Connor wants to change history and the new terminator ends up showing a window of opportunity.
T3 came along and stuff started to crumble. T3 makes no sense. A seemingly old terminator that goes back because a "more advanced" (and absurd) terminator came to yet again kill John Connor. They used preposterous ideas justified by the fact that it is a "fiction" film (i.e the stuff about controlling automobiles with a "virus": cars don't work like that). Well, even fiction must be based on some rules, and T3 simply disregards these rules, and that is what makes it so bad.
Salvation came and it was good. Honestly, I don't see why people freak out about Salvation, because it was consistent, it makes actual sense. It looks like a war zone between our present and that future we see in the first films. There are no crazy liquid-metal things this time, only "normal" cyborgs and there were very nice references to the first two films. I have no idea why people hate it. It is not a thriller, but action, but then T3 isn't a thriller either, nor is T2, so that is a very bad reason to hate TS.
And now came Genisys, with the typo in the name.
The film starts with a disturbing change: the judgment day occurred, apparently, in 1997. It means that the whole second film was thrown in the rubbish bin. Sarah's efforts were pointless, apparently Skynet wasn't destroyed after all. But maybe I was confused, so I'll just ignore this and move on.
The film introduces an interesting idea: since the future already changed before, Skynet was aware of John's plan to send Kyle Reese and attacks him (John) before he tears the time machine apart, altering the whole story.
But this leads to a series of confusing events that end up having a terminator sent to kill Sarah when she was 9 and another to protect her. Then the "good" terminator ends up "raising" Sarah to have the face of Emilia Clarke instead of Linda Hamilton and both prepare for the arrival of a disturbingly different Kyle Reese in 1984.
Then appears the original terminator, apparently unaware it was not the first assigned to kill Sarah, and a T-1000 was apparently sent to kill Kyle (it is never explained who was the target).
Then instead of waiting for the right time, Emilia Clarke decides to build a time machine and go to 1997 stop Skynet. But Kyle, as explained by some pseudo-science theory, has memories of an alternate childhood (go figure), and knows Skynet goes berserk in 2017. So they go to 2017 and find out that there is this software which is a mixture of Apple, Google and Facebook called Genisys that for no reason is being used on mobiles, hospitals, and even the army and is going to be "available" at precisely the time of judgment day, even though it is already being used.
John Connor shows up in 2017 looking like Jason Clarke, and we assume he was cast because he shares the same surname with Emilia, and he is a cyborg because Skynet itself was contained inside a cyborg and it managed to replace every cell in John Connor for nano-technology. He wants to turn his parents into machines and he inexplicably not only defends Skynet, but he created Genisys himself.
So John Connor is the bad guy! And he is a terminator who created Skynet! Schwarzenegger tries to kill him, but Sarah points a gun to his... head, and.. it makes no sense.
They then invade this company that developed Genisys and end up stumbling on the same computer interface used in Resident Evil: Skynet for some reason has several projectors in the place with the hologram of a child that tries to convince the heroes that they are doomed and you wonder why Skynet wastes time talking to them if it really thinks that, but the holograms make funny noises when they cross the light and you give up trying to understand.
In the end Schwarzenegger kills Jason Clarke by getting into this unfinished time-machine with a giant magnet, while Kyle and Sarah explode the building, but in the last second Schwarzenegger falls into the metal goo that would be the T-1000s and even though a T-1000 cannot emulate complex machinery because it has moving parts and all that (check T2), he inexplicably blends just fine with the goo and becomes "updated". Oh, that trick is used before too. So even though stuff exploded, the terminator "survives".
So the film ends leaving a feeling of "what the hell I just saw", and I wonder why people find this any better than T3 and why Terminator Salvation is considered worse. It is beyond me. Really people, something is wrong here.
So if you like Terminator, go see it. If you liked Salvation, I'm so sorry, but it didn't turn into a trilogy and they just "terminated" a good project.