In Half - Life: Opposing Force, the story unfolds as you play as an army grunt. You start off being sent into Black Mesa to deal with the situation. Along the way, you encounter various alien species that are a result of the experiment at the facility. You not only have to fight these aliens but also figure out how to survive in the hazardous and changing environment. There are different levels that take you deeper into the mystery of what really happened at Black Mesa, and you get to see the events from a different perspective compared to the original Half - Life game.
The story of Half - Life: Opposing Force mainly focuses on the soldiers in the Black Mesa incident. As a soldier, you face the chaos and strange alien threats. You have to fight your way through the infested facilities, dealing with both the alien creatures and the unforeseen consequences of the science experiments gone wrong.