There's no explanation at first, you just pick a weapon and move into the building, in which enemies (I'm pretty bad at 3D modelling, so it'll be robot-like things) are waiting and shoot anything that moves. For now, i just stick with the idea of having the player reach a certain point (computer or something) where he'll receive a clue or a bit of story, then he moves out where more enemies are waiting. Reach a certain point outside to end the level.
As for the buildings, a mix between industry/factories, offices, big halls, multiple buildings connected by a bridge. A good mix between open space shooting and quick corridor run-and-gun gameplay.
I guess I'll be tweeting some progress at @MarnixSuilen
, and perhaps blog some at aquatic-games.com
Anyway, good luck everyone and enjoy!