The mystery wasn't so much a mystery after like 10 episodes of hinting. I mean I had my suspicions from before but the lore drops in the later episodes and this one confirms it. If you don't know already watch this episode it'll make everything clear, and the best things is, is how artfully subtle they did it, without beating your head with a stick like you were Yuuji. There is something brewing under the surface of their tranquil lives, if you want to know what it is you will have to keep watching!