Half-Life: Opposing Force is an expansion for Half-Life on the GoldSrc engine, developed by Gearbox Software and released in 1999 for Microsoft Windows. The game was also finally ported to Mac OS X and Linux when Steam was released for those systems in 2010 and 2013.
Plot[]
After the resonance cascade disaster, the military initiates a cover-up operation to eliminate all alien presence at Black Mesa, as well as silence any witnesses. You play as marine Adrian Shephard, who is separated from his mates after his transport is downed by an alien aircraft. As the game unfolds, Shephard finds that the government's Black Ops team also intends to eliminate his unit to complete the cover-up.