You can check out the weapons available here: http://www.youtube.com/watch?v=XOoZEkR8i-A
The defining element of the SS games was the unrelenting waves of enemies that would come down on you with no cover at all. You'd be dodging back and forth firing constantly for minutes at a time before you cleared out an area.
As far as a larger plot, well, there really isn't one. Think Duke Nukem type of writing.