I kinda wish it didn't try to have a plot, because as it is, it's kind of a lame and boring one. Very few things can pull off serious plot and goofy dumb humour, and this definitely ain't one of those things.
lol it's fun. I started liking it much more when I realized it's pretty much just D&D.
Officially, there was a nuclear war (the mushroom war, I think it's called) and everthing got mutated. They explained it in a special.
But when I started watching the show on and off the first few times, my interpretation was that Finns father had created that world for him to go through and 'beat' to become...a man or king or whatever. Not sure how I would up with that impression, whether I pulled it from the show at some point or just my own head.