Originally this was a looser spin-off since there ware just light hints about this in OVA S3 and GXP, but now OVA S4 and S5 are tying it a bit tighter.
Was expecting something a bit more mediocre since Sasami Mahou Shoujo Club was a letdown as S3 and S4 for Tenchi Muyo! OVAs.
Anyway, links to main series during this are basically mentions from Kenshi without even citing names.
It's an old-school isekai, when the character usually had a reason to pop up in another universe and intends to go back when possible instead dying in our current and boring world and not even thinking much about it again.
But like in more modern ones, the mc is op to a fault, even tough one of the main reasons for this is all the training received before the beginning of the series.
Technically, it isn't a great work, the ost is forgetable, animation is on par with most animes (their CG isn't even as bad as some other isekais that released 10 years later), seiyuus are on point (even tough Rie Kugimiya isn't voicing a tsundere).
Story is fine, you need some degree of suspension of disbelief for a few things, but the intelligent characters try to come up with a few good ideas without trying to pull IQ250 bullshit and failing sometimes. Things go in a direction because characters need too, instead of simply happening. Comedy is abundant. There are a lot of characters, although most just appear, do their thing and that's it, no development, but at least they aren't forgotten as well.
Between action and comedy, the series is very enjoyable.
