Opposing Force takes the player through the Black Mesa Research Facilty in the eyes of a soldier who is sent to the facility to wipe out everything in the facilty.
Adrian Shepard is the soldier you play. The game starts out at bootcamp where mysteriously you are placed in advanced training. Once the training is over, you then are sent to Black Mesa where your helicopter is shot down by an unknown aircraft.
You wake up after the crash to see that a scientist has saved you from being killed. You must then fight your way through the facility to escape.