Kazuki Hoshino values his everyday life above all else. He spends the days carefree with his friends at school, until the uneventful bliss suddenly comes to a halt with the transfer of the aloof beauty Aya Otonashi into his class and her cold, dramatic statement to him immediately upon arrival:

"I'm here to break you. This is the 13,118th time I've transferred. After so many occasions, I have to say that this is all starting to grate on me, which is why this time I'm spicing things up with a proper declaration of war."

And with those puzzling words, the ordinary days that Kazuki loved so dearly become a cycle of turmoil and fear—Aya's sudden appearance signals the unraveling of unseen mysteries surrounding Kazuki's seemingly normal friends, including the discovery of mysterious devices known as "boxes."