Kei Nimori, a sixth-grade elementary school student, finds his own name mysteriously written on the classroom blackboard beside a mysterious title.
That night the school bell rings inside his room. Beyond the sliding door that suddenly opens lies "After-School", an otherworldly school shrouded in darkness.

Its classrooms are haunted by supernatural beings, while countless gravestones cover the schoolyard. In this nightmarish place, Kei learns that he has been chosen as one of the "Caretaker", observers tasked with documenting these terrifying anomalies...