You'd be pretty hard pressed to find a modern anime of any note that doesn't bundle in some goodies or other to try and tempt Japanese otaku to buy the DVDs and Blu-Rays, and this frequently takes the form of additional episodes that slot somewhere into the continuity. So it goes that some months after Sora no Woto ended, we find ourselves with the first of two extra episodes, in this case sandwiching itself into the middle of the series.
With this position in the middle of proceedings, it goes without saying that this isn't exactly a vital storyline to the wider series as a whole, no matter how much ominous portent is offered by the words of Kureha as the episode begins.
In short, this story is one of how Kanata learns about the secret manufacturing of alcohol going on behind closed doors with the other girls of the 1121st Platoon, after a chance meeting with Rio outside the supposedly jammed shut door which hides this operation. To deflect Kanata's questioning, Felicia spots an opportunity - One which naturally involves spiking the tea of the girls (including the visiting and ultra-religious Yumina), before suggesting that they all participate in a mock battle, with Kanata having all their secrets revealed to her if she should win. What follows is a slightly drunken contest which sees all of the girls taking things far too seriously, although the alcohol also softens the blow of any revelations to Kanata before the whole thing turns into an exercise in drunken debauchery. Not that that's always a bad thing, mind you...
So, with nothing of importance to add to the series, this bonus instalment of Sora no Woto is as filler-esque as it comes, alternating between being fun and slightly cringe-worthy while also teasing the fans with thoughts of things that they never get to see (although it's doubtless enough to get the dounjinshi makers working feverishly). As a ploy to get people to buy Blu-Rays I guess it might work, but if you can't be bothered to watch this episode then you really aren't missing anything, although a drunk Yumina is surprisingly entertaining in her own right now I think about it.