Original: The movie revolves around a "mutant cure" that causes serious repercussions among mutants and humans, and on the mysterious resurrection of Jean Grey, who appeared to have died in X2.
Simplified: The movie is partially based on two X-Men comic book stories: writer Chris Claremonts and artist John Byrnes "Dark Phoenix Saga" in The Uncanny X-Men (/O1980/O) and writer Joss Whedons and artist John Cassadays six-issue "Gifted" story in Astonishing X-Men (/O2004/O).