Half-Life: Opposing Force is the official game expansion for Half-Life, the PC CD-ROM named "Game of the Year" by several publications. In Opposing Force (Op For), players return to the Black Mesa Research Facility to experience an entirely new episode of single-player action. In addition, Op For contains a massive collection of new multiplayer content.
Remember the soldiers who gave you so much trouble in Half-Life? Now you are one of them--and life is not easy as part of the Opposing Force. As a soldier sent to eliminate Gordon Freeman, you are separated from your base unit. Amidst the chaos, you discover a new alien race that came to Earth through a portal that was opened when Gordon Freeman was transported to the border of the Xen world (see screen shots). It's quickly revealed that these highly advanced alien life forms have come to Earth to pillage the planet.
Although you were originally sent into Black Mesa to silence Freeman and his fellow scientists in the facility, you now find yourself stranded and confused. You are battling fierce new alien creatures and a variety of other foes in an effort to save your life.