Or so it thought.
Through no fault of the UAC, or its rival conglomerates, the hellspawn have returned, to wreak havoc upon the cosmos. This sudden incursion of demons is unlike any the UAC has experienced before. Billions of hellspawn are arriving through huge demonic portals and invading countless major UAC installations all across the quadrant, and Hell's branches are bringing forth new demonic creations from every corner of purgatory, which even the most powerful of the UAC's defenses can't overcome.
Due to the catastrophic loss of hundreds of brave marines at the hands of these hellish creatures, the once-proud military branch of the Union Aerospace Corporation has been reduced to just a few. You are a single UAC soldier, and now the only hope of holding off this demonic apocalypse. You have been instructed to ensure the total protection of the thirty-five major UAC facilities rapidly being invaded by the abominable forces of Hell. The odds are against you, and the demons are many, but now you have no choice. You must serve your overseers, or die.