On her way to a summer festival, Yuriko spots a mysterious woman on a bridge who suddenly disappears, leaving behind an envelope containing a diamond ring and what looks like a suicide note. After going over the contents with her teacher, Itsuki Isozaki, Yuriko, fearing for the woman's safety, decides to search for her with help from Utsumi and a reluctant Isozaki. She soon comes across Sakurako, who deduces that the woman's ring is mourning jewellery made from her late husband's bones, which she sought to send off to the afterlife after finding a new partner.