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 my 13,118th school transfer, and even I can't help but reach the end of my tether after so many iterations. So for a change of pace, I'll declare war this time." 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."