Half-Life: Opposing Force

You're trapped in a hostile research facility, fighting desperately for survival against overwhelming odds as alien horrors and ruthless soldiers close in from all sides. Every corridor is a potential death trap, and the tension never quite releases as you push through the chaos, outgunned and increasingly isolated.

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.