Worthington Laboratories--using a powerful mutant boy--develops a serum that eliminates the 'mutant X gene' permanently. This so-called 'cure'quickly divides the mutant community; Professor Xavier and his school are willing to give the government the benefit of the doubt but Magneto and his mutant Brotherhood see the serum as a vile threat to their way of life. They form an army of mutants and march on the fortified Worthington Laboratory located on Alcatraz Island. A much more dire threat appears in the form of the resurrected super-mutant Jean Grey, who has succumbed to her cataclysmic identity known as The Phoenix.