At St. Lucia Academy, the body of a male teacher has been discovered in the pool. The girl who found him, Mizuho Kousaka, is a shy bullied student who seems relatively unaffected by her discovery. After all, this isn't the first time she has seen a corpse. In order to teach Mizuho to stand up for herself, a girl named Shion Sonozaki befriends her and the two become inseparable, as they both share traumatic pasts. But when another body is found, Mizuho comes under suspicion for murder.