Kei Nimori, a sixth-grader, notices his name and the words after-school duty written on his classroom blackboard. He dismisses it as odd but pays no further attention until that night, when a school bell rings and a voice calls him to after-school duty. He is suddenly pulled into an alternate dimension of his school, where he finds himself alongside six others, including his former friend Sei Ogata. They are trapped inside this twisted version of their school from exactly 12:12:12 AM to 4:44:44 AM each night.

A mysterious force assigns them the task of documenting the school’s Unnamed Mysteries—entities that manifest as horrific monsters lurking in the classrooms and hallways. The students must observe, record, and survive these encounters, knowing that any mistake could cost them their lives. The mysteries draw on the traditional Seven School Mysteries of Japanese folklore, but here they are depicted as incomprehensible and violently real rather than lighthearted rumors. The story follows Kei and his fellow captives as they struggle to understand the rules of this nightmarish school, face escalating threats, and grapple with the tension between cooperation and self-preservation. The narrative arcs center on their desperate attempts to document each mystery, the slow erosion of trust and sanity, and the hidden connections between the monsters and the children’s own pasts.