A three-level (but pretty short) solo scenario. There's a plot, but it's sort of irrelevant... the game is focused pretty tightly on action. It's difficult-the first save term is most of the way through the second level. (At least the first one I found.) Map view helps (some of the areas are labeled, and the labels can save you a bit of headache later). There a few secrets, but mostly, it's just "figure out how to kill 'em all before they kill you" carnage. Definitely worth the runthrough.