The solitary island of Kyuubi-jima lies in the Sea of Japan. It was named after the legendary nine-tailed fox after a shard of the sesshouseki (killing stone) had reached the island when it died. However, many people left the island to escape from the inauspicious name, and before long the island had its name changed to Hotarubi-jima (light of the fireflies). However, even with its new beautiful name, an ominous shadow still casts over the island.
Natsuki lost his mother when he was born and moved at a young age to Hotarubi-jima when his father remarried, along with his two new imoutos. But his surroundings changed completely after the accidental death of his father. Taking care of three kids was deemed too much for his sickly mother-in-law Kaede, so Natsuki and his youngest sister Tsugumi were taken in by relatives outside of the island, while his other sister Madoka remained on the island as the successor of the Tsukumo household.
Many years later, Madoka suddenly vanished and there were whispers among the islanders that she was spirited away. No one could find her, so she was considered as dead by an accident. News of Madoka’s death reached Natsuki and Tsugumi on the mainland. However, Kaede prohibited their return to the island and they were unable to attend her funeral. A few years after that, Tsugumi suddenly received a letter from Madoka, simply telling them to come to the island. After discussing it with Natsuki, they decided to return to Hotarubi-jima to find out the truth behind the letter. They soon learned about the sad truth and cruel past of their family clan, as well as the secret of the island.