Entirely new addon episode for
Half-Life. In Half-Life, you play the role of Gordon, a
scientist who has to escape from the alien-infested
Black Mesa research facility and save the world from aliens. In Opposing Force, you play the role of Sherman, a marine who is sent into Black Mesa to kill Gordon before he can tell the world what's happened.
Opposing Force even surpasses Half-Life in terms of coolness. The story is simply amazing. Best story of any game I've ever played. And the gameplay is even more amazing...it's almost like being inside a movie. The character AI seems to have been improved even more from Half-Life as well. I highly recommend this game.