The X-Men are mutants–born with extraordinary powers that set them apart from the very humanity they are sworn to protect. Now, ground-breaking research has revealed a method for reversing the genetic mutation responsible for mutant powers–whether the mutant wants it reversed or not. The U. S. government wants to use this procedure to eliminate the so-called mutant menace once and for all–but the X-Men soon learn that the true source of the procedure is their old enemy, Sinister! And his agenda is far more complex and dangerous than even the X-Men can imagine.