This is an OVA. It is actually 'stand alone' and has little to do with the main series. This presents itself as one of those 'ever popular' historical period re-envisioning episodes, and seems to use the 'personalities' (more or less) of the characters at the end of the series- but not really. Long story short, this OVA is constructed like a filler episode, is completely disconnected from the series, and probably isn't worth watching unless you're a completionist.
In addition to the characters being 'off', the plot is weak, and resolved multiple times by what I assume are blatant and intentional Deus ex Machina resolutions.
You my find the nonsensical/quirky outcomes humorous (I think that was the intent), but in comparison to the original series, this episode is a bit of a let-down to say the least.
I don't know why they made this. It's not horrible, but it's not very good either. It was probably created as 'light-hearted', intentional nonsense- but doesn't quite pull it off.
Watch it if you want, just know it presents as a 'proper (but bland) story' for about 2/3rds before spiraling into insanity/obscurity. There is a reason the main series is ranked around 500 (good), and this one ranks in around 1700 (nowhere near as good). The reason is they are two completely different anime.
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I NEVER write reviews but after reading the other one I felt I should put a little input.
This OVA was overall just fun to watch; the whole time switch thing was iffy but opened room for randomness which had me laughing out loud a few times. Up until the half way point I was loving this, the way they were doing things was really working; I was thinking 10/10, but then things really went down hill and made you look back and think "was this really the whole plot?" and you look at it completely differently. The very end was quite strange and you wont
see it coming from a mile away, it redeemed itself somewhat from the last minutes leading up to it but overall was not an ending I was hoping for, though the ending would have been superb if the conclusive ending was so stupid.
Overall 8/10, it was great starting out, ruined by a terrible plot, then partially redeemed with the final scene that should have been the plot the whole time.