遺書、公開。

In the spring of a new semester, a "ranking" is sent to Class 2-D. It clearly shows the rankings of all the students and the homeroom teacher. The one who ranks 1st is Himeyama Tsubaki, a kind and popular honor student recognized by everyone. But one day, without any warning, she commits suicide. A few days later, everyone in the class receives a suicide note from Himeyama, and from that day on, the class begins to fall apart.