X-Men: The Last Stand is the sequel to X-Men: First Class, X-Men and X2: X-Men United.
The film recreates the famous Dark Phoenix Saga, and also the "Gifted" story arc from Astonishing X-Men, which involves a cure for mutants.
The X Men discover that Jean Grey did not perish after the escape from Stryker and the Weapon X program.
They go to investigate only to find her greatly changed.
She joins with Magneto and his brotherhood in their attack on Worthington Industries, makers of the cure for the mutant gene.