It's based off an infamous 1981 Uncanny X-Men storyline originally created by Chris Claremont, John Byrne, and Terry Austin. It told of a dark future where Marvel's mutants were being rounded up by robotic Sentinels and placed in concentration camps to await execution. The future X-Men telepathically transported the adult Kate Pryde into the mind of her present-day self. She then had to convince the present-day X-Men to help her stop Mystique from killing Senator Robert Kelly, whose death will jumpstart the anti-mutant hysteria that leads to the Sentinels and mutant concentration camps.
Now replace the underlined words with adult Kate Pryde,Wolverine, young Charles Xavier & Magneto, and Bolivar Trask and you have the basic storyline for the X-Men: Days of Future Present movie. The rest are just details -- and there are a lot of great details! Such as...
1. Quicksilver: They introduced a young version of Quicksilver in this movie and did a great job of portraying his powers in action coming from his own perspective.
2. The future X-Men were pretty kick-ass. They consisted of Kate Pryde, Bishop, Sunspot, Iceman, Colossus, Blink, and Warpath. They have a pretty cool method of surviving. Warpath keeps watch for the Sentinels once their bases get noticed. Kate then uses her phasing powers to send Bishop's mind back in time a day or two to warn the X-Men of the attack. Blink finds them a new home and they end up escaping the Sentinels before they arrive.
3. Pretty much everyone from past movies returns -- if only for a cameo. That includes old Xavier & Magneto; young Xavier & Magneto; Storm; old Beast; young Beast; Havok; Jean; Cyclops; Rogue; Havok; etc... I think the only X-Men who doesn't make an appearance is Nightcrawler.
I have a confession to make. I have loved pretty much all of Marvel Comics' Avengers-themed movies. But I've watched the X-Men films more out of loyalty. There are usually enough bits to keep things interesting and I admit that I enjoyed the previous film more than the others, but this hasn't been my favorite film franchise. But I really liked X-Men: Days of Future Present. It was great story, character movement, adventure, and believable special effects. Those factors all combined effectively in this movie.
This is definitely a movie to check out.