Page depicted Pryde in that third "X-Men" movie, "X-Men: The Last Stand," and her casting in "X-Men: Days of Future Past" means that Singer is keeping the continuity from all the movies, including the ones he didn't direct, which is pretty cool.
The plot of "X-Men: Days of Future Past," is based on a two-part comic book arc by Chris Claremont, John Byrne, and Terry Austin, that was published in 1981. In the arc, a post-apocalyptic future is avoided when Page's character, Kitty Pryde, transfers her mind into her younger self (a modern-day Kitty Pryde), in an effort to prevent the impending disaster.
It's unclear how the new film will reconcile this plot line, considering the two timelines will be made up of some kind of '70s X-Men group as well as the "modern day" bunch -- which is why Patrick Stewart and James McAvoy appear side-by-side (along with Michael Fassbender and Ian McKellan). It should be noted that other returning cast members from both threads of the franchise include Hugh Jackman, Halle Berry, Jennifer Lawrence, Nicholas Hoult, Anna Paquin, Shawn Ashmore (who you can see in Singer's photo too), and Daniel Cudmore. Additional cast members include Adan Canto and "Game of Thrones" favorite Peter Dinklage.
"X-Men: Days of Future Past" will confuse us all on July 18th, 2014.
[h/t HuffPost Entertainment]