Shotaro and Sakurako deliver Sasaki's belongings to his sister, Sayuki Haruma, who explains how Natsuko was her caregiver and close friend to her and Sasaki. After Natsuko had a premature birth, resulting in the death of her child, she left the household and passed away a few years later, with her bones accepted by Sayuki. As Shotaro hides a photograph, Sakurako follows clues leading her to where the baby's bones were buried, which also contains one of Natsuko's bones that Sasaki had put in with them. Returning the bones to Sayuki, Sakurako deduces from the shape of the toe bones that Sayuki is the baby's actual mother. After understanding the full story, Shotaro goes back to return the photo to Sayuki, who deduces the poem on the back was a farewell message from Natsuko. The next day, Shotaro confronts Sakurako about the stolen cat bones, deducing they were from another cat she owned and was a former alumni at his school. Sakurako confirms this, explaining how she brought both her cats, who died of poisoning, to Sasaki, who made them into skeletons and taught her all about osteology.