To this day, I have no clue what this game is about, except you need to kill everything, in the fastest, flashiest way possible, without getting owned yourself. Even if something doesn't seem to be hurting you, it probably is, so just kill it.
Is there a back story? Yes, but I don't care. I don't think anybody does, really. Everything is randomly generated every time you play, so you'll probably not hit the same giant whatsit every time you die. You'll die all the time, honestly - you're supposed to get a higher score each time, but that never happens. Or maybe it does, but I'm too busy looking at all the funky colours.
Really Big Sky is a massive time sink, if you let it go there. You'll constantly try to do better and get farther than last time, and be blown away anew every time there's some new whatsit on screen. The music is this intense thumping rave stuff that will drive you bonkers, but it suits this title so well. Visuals are sharp, bright, intense, subdued, and everything in between, and usually all at once.
I highly recommend this title. It's stupidly good fun, for a ridiculously low price, and it runs fantastically well to boot.