Feels a bit rough around the edges in terms of setting up viewers' emotional investment, which kind of makes it feel more like filler than a prologue. Not sure why that is... maybe it's because we get this intro conflict with the aliens and then there's no hint that the protagonists are going to get caught up in it except that it's there (oh.. and maybe that obscure sarcastic "Nothing in the universe is going to stop that" at the end... but let's ignore that for a minute). However, that isn't exactly a new thing... nor a bad thing... plenty of big budget cartoons pull the "Here's a thing, but our dopey protagonists don't give a shit about it yet!" So, really, maybe it just needed something more to identify that the protagonists are gonna get involved with the aliens -- maybe putting one of the evil aliens from the first scene into the arena ad would have worked... or something similar depending on what your actual script is like; also I think maybe a more obvious tone in the line-delivery could have helped to make it more OBVIOUS that the ALIENS would INDEED somehow get INVOLVED with THE protagonists.... MAYBE *cough*.
But anyhow, I'm just a Newgrounds reviewer, Jim, not a scriptologist or phonetician; the rest of this is obviously top notch enough to warrant 4 stars despite the deeply troubling feeling of non-investment that I get from this filler.... uhhh... I better make that a 3.5