Instead of a prologue, I would treat it as an extended scene from that episode. If one is curious about how the battle looked from the beginning and wants to briefly meet some new characters, they are welcome to take a peak. In the end, it's barely 12 minutes of runtime.
Watching this between episode 2 and 3 of the TV series seems optimal, though not necessary for changing the overall enjoyment of the series.
It's hard to treat it as a standalone piece. An inoffensive animation showcase that was just fine. (6/10)
