Koyomi finds himself trapped in a phantom classroom with Oshino Ougi, apparently stuck there with time frozen due to the effects of another Oddity. Ougi determines that they are trapped there because of a similar incident that happened two years ago, when Koyomi was a high school freshman, and marked a turning point in his life. He recounts the incident, which involved an alleged leakage of the answers to a maths exam during a study session, and how a classmate of his, Oikura Sodachi, was determined to find the culprit. Among the suspects are those who scored highest, which included Koyomi and Sodachi themselves, and the argument is heated until the matter is put to a vote, and Sodachi is herself chosen as the culprit. She never came to school again since. Koyomi however, personally believed that it was their maths and homeroom teacher Tetsujou Komichi who was responsible, adjusting enough of the questions in her own exam to match the study session. That she herself voted for Sodachi made him lose faith in people and made him ever more withdrawn and cynical since. This allows Koyomi to leave the classroom.
The next day, Koyomi comes to class, and is warned by Hanekawa not to enter the classroom, as Sodachi has suddenly returned to school.