Darknation's Wacky Adventures in Impse or Dnwaii (pronounced 'denwhy')
Okay, in reality there's no story. I had a bunch of textures, and I had a bunch of imp sprites, so I smashed them all together and here's what came out. It's two maps, btw. You don't have to read any more if you don't want (though I doubt most people have even gotten this far).
The Real Story: You are Darknation, secret agent guy for the government of some country nobody gives a shit about (France or Norway or something). Some big government guys who probably wear fancy suits and dark sunglasses (and those thingies in their ears, radios or cell phone dealies or whatever) have just gotten word that a bunch of experiments have gone horribly, horribly wrong in some undisclosed location (never saw that coming, did you?). Your mission, should you chose to accept it (you don't have a choice anyway), is to infiltrate the base and kill the leader imp dudes (you'll know when you beat them because the map will end).
Sadly the only way you could get into the facility was via one of those crates the UAC is famous for. Once you bust out of it you should be good to go. It's time to kill some god damn dirty furries! Oh, I mean imps.