The final chapter in the "X-Men" motion picture trilogy. A "cure" for mutancy threatens to alter the course of history. For the first time, mutants have a choice: retain their uniqueness, though it isolates and alienates them, or give up their powers and become human. The opposing viewpoints of mutant leaders Charles Xavier, who preaches tolerance, and Magneto, who believes in the survival of the fittest, are put to the ultimate test - triggering the war to end all wars.