Half-Life: Opposing Force

Opposing Force is the first official expansion for Half-Life, developed by Gearbox Software under the supervision of the original creator Valve. The storyline of the base game is interwoven into the add-on, as players take on the role of Corporal Adrian Shephard, one of the soldiers sent to the Black Mesa facility to "clean up" the incident and silence any witnesses.

Shephard never reaches his final destination, though: his helicopter is shot down, his squad killed, and he finds himself in a battle for survival against the invading aliens.

Opposing Force takes place concurrently with Half-Life, and Shephard will witness some of the same events (though from a different perspective) and visit some of the same places as Half-Life's protagonist Gordon Freeman did in the original game. The add-on uses the same style of storytelling as Half-Life: everything is seen from the eyes of the protagonist.