The story of the game takes place in a science research facility in New Mexico named Black Mesa. In the game, you play as Gordon Freeman, a scientist who works at Black Mesa. A test that Gordon helps with goes wrong, and causes a "resonance cascade." This causes portals to appear all over Black Mesa. Aliens from a different dimension come through the portals and kill many of the scientists and workers. The dimension that the aliens come from is called Xen. After that, the United States Military is ordered to kill all the scientists and aliens in Black Mesa because they didn't want people to know about the accident. Gordon must fight aliens and marines on his way through Black Mesa, then teleport to Xen so he can kill the alien leader.