Set during the classic Claremont-era X-Men, an alien race called the Kh'thon returns to Earth claiming ownership of the planet, and of mutantkind, whom they say they engineered as servants generations ago. Backed by Exemplars bred to match each X-Man's power, the Kh'thon force Wolverine, Kitty Pryde, Nightcrawler, Cyclops, and the rest of the team to split up and call in X-Force to keep humanity from being claimed as property.
Summary by Loreful, from public sources