"Hooper started with the plot. "All hell's broken loose again," as he put it. Two years after Doom 3's ending, Earth is receiving a strange signal from Site 1 on Mars, one of the UAC sites you don't visit in the first game. Dr. Elizabeth McNeil, who you might remember as Dr. Betruger's right-hand woman, is sent to investigate, and you're a Marine combat engineer sent along as escort."