On the first day of the school year, the incoming students of Class 2-D at Kairei Private Middle School receive an anonymous message ranking them. No one knows what the ranking means, but the students quietly reach a consensus: it is a pecking order. At the very top of the list stands the good-natured Tsubaki Himeyama, but her unbiased kindness soon makes the ranking seem irrelevant.

However, tragedy strikes when Himeyama commits suicide in the girls' bathroom. Three days after Himeyama's death, her dazed and grieving friends return to a chilling sight in their classroom. On each desk—including the teacher's—is a different suicide note by Himeyama, 29 in total. Determined to understand why someone so flawless would take her own life, the students begin to share the contents of their individual letters every week. But each of them has a secret to hide, always circling back to the strange ranking and its dark consequences.