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Doom 3 wiki article:
http://en.wikipedia.org/wiki/Doom_3
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Expansion article:
http://en.wikipedia....rection_of_Evil
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the UAC(the company to blame):
http://en.wikipedia....ace_Corporation
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Gameplay vid 1:
http://www.youtube.c...feature=related
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Gameplay vid 2:
http://www.youtube.c...feature=related
(both are kinda of bad sound quality, and aren't really the 'good bits', just the starting off the game before the demonic invasion, but it gives you an idea of the setting)
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Basically, UAC has a base on mars, they make all sorts of advanced stuff, like teleporters, all hell breaks loose, and demons invade through these teleporters. The Doom Marine is stuck in the middle of it all, he goes to hell and back, killing the big cyberdemon among others, Ending the demonic invasion... or did it?
In the expansion you're a different marine who goes in after the events of the first game, when efforts are under way to reclaim parts of the facility and get things going again. You find an artifact, which then brings the demonic hoard coming after you anew. You go through different sections of the base, and even returning to some of the sections you saw in the original(which are still crawling with demons).
In the end you find the old boss of the place, who brought the demonic invasion in the first place and is working with the demons(you find this out in the first game), and he is part of some sort of flying demon skeleton thing. You kill him and stop the invasion again.
Edited by some_weirdGuy, 07 March 2009 - 06:41 AM.