| Spoiler warning! This section contains plot details about the novels. |
The Rejecting Classroom was a box that Kasumi Mogi had wished for in Volume 1. The Rejecting Classroom repeated the same day, where Maria Otonashi transferred into Kazuki Hoshino's school, for tens of thousands of days. Within the box, every student forgot their memories each day and saw everything as normal everyday life. There were, however, four people inside this box that could remember; Kasumi Mogi, who was the owner of the box, Kazuki Hoshino, the main character, Aya Otonashi (later being called Maria), the main heroine, and the mysterious entity, O.
As the box's owner, Kasumi could remember all events that happened within the Rejecting Classroom, and kept her ownership a secret. The reason that Kazuki could remember are not entirely specific, however, it could be implied that the reason was either because he had longed for an everyday life, or because he had come into contact with "O" and boxes before. Maria was later revealed to have been a box herself, which was also the reason that she could remember the events of each day in the Recjecting Classroom. The entity "O" was actually the one who gave out boxes, and used the Rejecting Classroom to observe Kazuki.
Kasumi Mogi, after 27,755 loops, decides to commit suicide, in an attempt to destroy the box, but not before killing Aya & even Kazuki himself. However, "O" is dissatisfied and wants to keep observing Kazuki, and therefore, instead of ending the Rejecting Classroom, the entity "rejects" Kasumi, replacing her with Maria. The rest of the class, and Kazuki himself, was convinced that Aya was Kasumi and saw the situation as normal. Maria, who approached Kazuki, and eventually everyone in Kazuki's clique to reveal herself as Aya Otonashi.
One of his dearest friends, Haruaki Usui was revealed to be "O", who filled his place after the former had become rejected by the classroom. "O" strikes a deal between itself and Kazuki and Maria, giving them the box in return for letting it go, as they had threatened "O" by claiming to "erase" it. The two are then sent into the box itself, with "O" disappearing, only to find Kasumi in despair.
Upon finding Kazuki in front of her, Kasumi sees this as a rescue, although Kazuki himself sees this as trampling her wish, even as she thanks him. Kasumi bids him farewell as she accepts her fate. It is revealed that Kasumi survived the accident, and is seen in hospital, her memories of the Rejecting Classroom had seemingly remained, however Kasumi herself believed that it was merely a very long dream.