Duke has to save the world once again from the onslaught of aliens that decide to attack Earth. Compared to the past Duke games, Duke3D is a bit darker and more violent, although its violence is pretty tame compared to today's standards. There are even parental controls, but everyone knows that parental controls are for pussies.
Playing Duke with DOSbox isn't recommended by me. If you feel like doing it for the hell of nostalgia, go ahead- but the most fun can be absorbed if you use a source port, mostly preferably EDuke32. A lot of mods can also bring a more modern feel of gameplay that you're used to, so check out the mods that GOG has listed on the forums. Two mods that I recommend are Duke Plus and Weapons of Mass Destruction: the others I haven't played extensively so I won't bother mentioning them. If you're looking for simple fun without a massive storyline, then Duke is for you. There's no reason to not give it a try.