It's CABLE vs. DOMINO! The frequent partners take opposite sides on altering the time stream to prevent Harmony and the Ranshi Empire from ever existing.
Cable has to either stop or allow the murder of a powerful mutant who is possessed with an alien being. His desion will result in two diffent futures both paradise in differnt ways. The Assassin, Domino also under the control of an alien being. Cable stops Domino from killing the mutant yet he tricks the mutant into using his powers of emotion control to make the world hate him resulting in making both futures cease to existence.