Mouryou no Hako: Chuuzenji Atsuko no Jikenbo - Hako no Yurei no Koto

In essence, this OVA is basically an explanation for those who didn't understand what was going on in the original series; although, it doesn't help much. It works very much like a summary, only it takes place from the point of view of Akihiko Chuuzenji's sister: Atsuko Chuuzenji. It's only 15 minutes long, which is short even for an OVA, and if you've seen Mouryou no Hako and didn't understand anything; this could help you. It also seemingly takes place between earlier episodes (as evident with the car scene with Sekiguchi), but also in the present. In short, this OVA is all over the place.

This episode, you could say, is almost like a summary, which is why I gave an 'Overall' of 6.

Due to the complexity of the main anime, and the murders as well as the 'supernatural perspective' in it, it was quite hard to comprehend what was going on at the beginning.
However, at the end I'd say I understood 80% of what had happened as everything was revealed then.
But what about the remaining 20% which most of the watchers, including me, couldn't comprehend or had failed to remember?

Well that's when this Special works its magic on us.

It basically retells the events that took place ... in the story from the perspective of Atsuko, the younger sister of the talkative priest Akihiko Chuzenji, who is seen in one of the earlier episodes - this was a strength to the Special as they reused a very minor character so that we at least have a connection to them, a contrast to a few other anime in which random, unknown characters are used to do this.

Furthermore, the Special has a story in itself which links to the telling of the murders that took place, which doesn't make it a pure summary - I found this interesting as I could then not doze off but instead I was interested in it.

The Special also made it so that the episode itself was centred around Atsuko's perspective from day 1, during the time when the murders were happening, and ended itself a year later or so after the case was solved - this was indeed a great element to it because the Special doesn't take place 'after' the anime's story takes place, but during, which made it all the more realistic and not just 'forced'.
In addition, they include one of the scenes from the earlier episode in the main anime where Atsuko was in just to mix it in to make it seem more natural and to also remind us of who she is.

I also loved how they reused the soundtracks and characters, it was almost nostalgic.

So to conclude, definitely watch this if you didn't catch on to all of the details of the main series.