An alien race known only as the Kh'thon appears in the skies over Earth, claiming that they are the former occupants and rightful owners of the planet. Their ships are manned by baseline Homo sapiens, supposedly descendants of the early hominids who served the Kh'thon on Earth, countless millennia ago. The will of the Kh'thon is enforced by the Exemplar, humans augmented with a dizzying array of powers, each of them the match of any one of the X-Men. The Kh'thon claim to be responsible for the presence of the x-gene in human DNA, included so that they could breed whatever traits or abilities they required of their servants. No one is sure whether to believe the aliens' claims or not, but given the amount of firepower at their disposal, it hardly matters one way or the other. It is up to the X-Men, working in uneasy collaboration with Royal Marine Colonel Alysande Stuart, to find a way to repel the Kh'thon invasion of Earth, before terrestrial humans and mutants alike are enslaved by alien masters.