A fateful encounter between a nurse from the hospital in the port of Mojiko and a tourist from Taiwan.
Fukayama Natsuki was on her way home from work when she came across the unconscious Hayashi Mirei surrounded by a worried crowd. “I finally found you…”, the now conscious Mirei spurted in Mandarin and suddenly fled the scene with Natsuki’s hands in hers.
As they fled, Mirei revealed that she had to be somewhere that day and that she needed Natsuki’s help in finding it. Natsuki, worried about Mirei’s health, obliged under the condition that they would see a doctor to have Mirei checked. However, it was then that Mirei showed Natsuki the only lead she had of the place — a single photo of a young girl smiling back at the camera against a backdrop of a peculiar cavern.
Where is that cavern? Why did Mirei have to be there that day no matter what? Who is the girl in the photo? Their seemingly chance-like encounter will slowly twist and tangle into a fateful one…