It's very difficult to explain Terminator Genisys. This is partly because it is the fifth Terminator film (following on from Terminator Salvation back in 2009) and really only makes sense if you have seen the previous installments and also because, although it does initially follow on from the fourth film (with the odd inconsistency), it then completely re-writes the entire history of the Terminator universe.
The basic premise of the Terminator films (to the few who are unfamiliar) - which span back to The Terminator in 1984 - is that, in 1997 a computer program known as "Skynet" becomes sentient and, deciding that humanity is a threat, deploys a large amount of bombs which kill a significant proportion of the world's population. The date when Skynet becomes self-aware is known as "Judgment Day" (as is the second film). Skynet then creates humanoid robots known as terminators (some of which look human) whose purpose is to kill all remaining humans. Salvation was the first film set post-Judgment Day.
In Genisys, the human resistance is near victory and John Connor (Jason Clarke), the protagonist in many of the films and the leader of the resistance, sends Kyle Reese (Jai Courtney) back in time to 1984 to protect John's mother, Sarah Connor (Emilia Clarke), from a terminator (Arnold Schwarzenegger) who has been sent back by Skynet to kill Sarah and prevent John from ever being born. Complicated right? When Reese arrives in 1984 with instructions to protect Sarah, he finds she is waiting for him, aware of his task and with Schwarezenegger's terminator as her ally. This is where the plotline deviates from the original films.
The first three Terminator films had very similar stories, in that each saw either John or Sarah Connor on the run from a single terminator who had been sent back by Skynet before Judgment Day to kill them. The war is yet to happen in these films and they are best described as thrillers. The fourth film was more of an action film and was set mid-war and in my personal opinion, is nowhere near as good as the first and second film. The third film was a bit of a dud but the first and second are great films (and Judgment Day has aged incredibly well), full of suspense and, at times, quite frightening. The best thing about these films was the characters: they both had great villains (Arnie in The Terminator and the amazing Robert Patrick in Judgment Day) and the heroes, Kyle Reese, John and Sarah Connor, were strong-willed and likeable.
Genisys is also set mostly pre-Judgment Day and sees its heroes, Sarah Connor, Kyle Reese and "Pops" (Sarah's name for her terminator protector), face for the most part one main villain, a new and improved terminator (try to avoid the trailers which give ridiculous spoilers on this). Because it goes back to the old formula, I much preferred Genisys to Salvation. It felt much more like a Terminator film for me and I do love Arnie in that role (he barely makes an appearance in Salvation). The new villain is quite cool and, although it's another terminator, it’s unlike the terminators we've seen before. That said, Genisys is not as scary or as suspenseful as the first or second films.
I really enjoyed Genisys and thought it was a good action film, but I do think it has its faults, in particular that its characters generally aren't as well-developed or as cool as in the earlier installments. Arnie fits into his old role like a glove and Emilia Clarke is likeable as Sarah Connor. However, Clarke is nowhere near as bad ass as Linda Hamilton's Sarah Connor from Judgment Day who, let's be honest, is one of the coolest female leads in any film. Those are pretty tough shoes to fill to be fair! Courtney is quite generic as Kyle Reese and, I feel, could have been replaced by a number of other handsome, buff men without any detriment to the film (not so much his fault perhaps as the writing). The emotional scenes in the film, particularly between Sarah and Kyle, seem quite forced, although I did like the relationship between Sarah and Pops.
The ending is fine and I would not say disappointing, but it is nowhere near as epic as the finale of The Terminator or Terminator Judgment Day. The latter has one of the most memorable movie endings ever!
Overall I did enjoy Genisys. I love the Terminator franchise and would be very happy to keep watching more films. That said, the first two films are so good, especially Judgment Day, and I don't think any new film will ever meet that high standard.