[geRV]

Originally Posted by WidowMaker555

I watched it all but i still dont get half life 2 how the alien people are on earth...some1 explain

In halflife 1 a portal to an alien planet was opened at the black mesa facility which was a research station, basically its assumed that the combine has control of that planet and they use some of the aliens that make the zombies (the headhumpers) as part of their attack on earth.