Basically the backstory of how all the main characters in Kyoukai no Kanata met. This may be the prequel to the series story wise but I would watch Kyoukai no Kanata first. Everything will make much more sense in this if you watch the main series first. Plus some of this may be considered spoilers to events in Kyoukai no Kanata. The story mainly focuses on the Nase family, Hiroomi and Mitsuki in particular, and how they become involved with Kanbara. The majority of the other main characters from Kyoukai no Kanata are in this special as well. I always enjoy seeing backstories for characters, and thisspecial did just that exceptionally well. It was never explained how Hiroomi and Mitsuki met Kanbara in Kyoukai no Kanata which was nice to finally see in this special.