The comic book world is so expansive that, unlike a novel series, there is no one story or plot that must be followed. Comics allow for time travel, alternate universes, immortality, resurrection of dead characters, and even allows characters to have different incarnations. This helps a smart comic book writer get past continuity problems between films without having to reboot the whole thing. Not counting last summers The Wolverine, we've had 5 X-Men movies from four different directors and even more writers. So, how could X-Men: Days of Future Past Director Bryan Singer bridge the gap between all of these and make tie them all together? I give you the time travel/alternate universe element.
In X-Men, X2, & X-Men: The Last Stand we had a cast of characters the comic book fans loved and the end of X2 gave us a taste of where he wanted to go with the series. However, he was given the opportunity to direct his dream film Superman Returns and he bolted the X-Men franchise leaving 20th Century Fox to bring in Brett Ratner to direct X-Men: The Last Stand which was a commercial success but did not satisfy the massive comic following and left the X-Men franchise in limbo. Director Matthew Vaughn was brought in to reboot the franchise with X-Men: First Class which was an origin tale of Magneto and Professor X and the formation of the X-Men. It gave us a new cast and introduced beloved characters while leaving several out. In between those films, director Gavin Hood gave us X-Men Origins: Wolverine which (as you can guess) focused on the origin story of fan-favorite Wolverine. This movie was originally to be the launching pad for several X-Men origin films but it wasn't well-received by the comic-loving public. Last summer we had another Wolverine movie, which was a good film but didn't have a direct tie-in with the X-Men films.
So, how do you bridge the gap between these films? Instead of rebooting the series again, you create a film that deals with time travel and alternate realities in which anything is possible. You make a story about an apocalyptic future in which mutants, humans who might mutant children, and humans who help mutants are hunted down by powerful robots who can adapt to any mutant power. In this future, you use the actors and characters from the first X-Men trilogy and throw in some new ones. You bring in Ian McKellan as Magneto, Patrick Stewart as Professor X, Hugh Jackman as Wolverine, Ellen Page as Kitty Pride, Shawn Ashmore as Iceman, and Halle Berry as Storm. You tell the audience that the only way to save the future is to send Wolverine's mind into his past body, which Kitty Pride can do through her Mutant powers. So you send Wolverine back to the past and encounters the main (younger) characters from X-Men: First Class. You bring in Michael Fassbender as Magneto, James McAvoy as Professor X, Jennifer Lawrence as Mystique, and Nicholas Hoult as Beast. Wolverine has to convince his friends in the past that they must stop Mystique from attempting to kill the weapons designer who uses her DNA to create the machines (Sentinels) that cause his apocalyptic future.
The best part about it? It works. This is not an action movie that many will expect. The action is fine and picks up right from the beginning but the storyline makes it interesting. Many subplots that tie in with the X-Men universe that fans and audiences love. It's rarely confusing and it doesn't stray from its' main theme. Change the past, save the future.
This is the best comic book movie acting ensemble ever assembled. I could watch the Magneto's and Professor X's on-screen all day long. Hugh Jackman brings his usual charisma to the character and when you add in Jennifer Lawrence and Nicholas Hoult it becomes something from which you can't look away.
You really need to be familiar with the original X-Men trilogy and a little familiar with the William Stryker character from X2 and X-Men Origins: Wolverine. You can get by without having seen them but you should read a synopsis or ask one of your geek friend to fill you in.
It's a very good comic film on many levels just don't get too caught up in trying to make it work with every plot line of the rest of the X-Men franchise.
Stay for the post-credits scene. Look up the character Apocalypse when you get home. The next X-Men film is going to be awesome.