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.