Aware of an impending assassination which will create a catastrophic future, the X-Men must deal with threats both outside and within their team.

Rushing to investigate a disturbance downtown, the X-Men find Sabretooth on the rampage again, but realize too late that it isn't really Sabretooth at all. It is the Phalanx, a voracious alien life-form that can assume the guise of anything or anyone. Before long, the X-Men are all captured, except for Beast, who must find a way to stop the alien horror from devouring the entire Earth.

Snatched from the jaws of the Phalanx by the villainous Mr. Sinister, Beast and Forge search for allies to help them battle the alien threat. Gathering assistance from such unlikely sources as Magneto and Sinister himself, they embark upon a daring and risky plan to free the planet from the clutches of the Phalanx by traveling straight into the belly of the beast.