Half-Life: Opposing Force Developer: Gearbox Software Released: Sierra Entertainment 1999 Source port used: Steam version
** Plot Opposing Force returns to the same setting as Half-Life, but instead portrays the events from the perspective of a U.S. Marine, one of the enemy characters in the original game. The player character, Adrian Shephard, is sent in to neutralize the Black Mesa Research Facility after a scientific mishap causes it to be invaded by aliens, but quickly finds that the Marines are outnumbered and slowly being beaten back by a second alien race and black operations units.
** Notes The Steam version is the closest to the original that I could find, although the initial menu and loading screen are different. The Training Room tutorial is appended to the video.