There were four fourth-graders who were good friends on Koshiki Island. Abe Takashi, son of a fisherman, Ishidaira Kyou, who is good at studying and sports, Nakahara Kou, who just recently transferred from the mainland, and everyone's idol girl, Yamada Miyuki. When Hoshino Ayaka, a nurse at the clinic, tells them that they could see a star-filled sky in a cave at night, they snuck out of the house without telling their parents and infiltrated the cave. There, they were able to see a starry sky that looked like a night sky filled with countless twinkling noctilucent insects. But then the tide comes in and the cave is flooded, and the seawater keeps coming into the cave. If nothing is done, the entire cave will be submerged. Will the four be able to survive?