Living on a small island slightly removed from the Japanese mainland, Yuzuriha and Guest grew up together like siblings and were irreplaceable best friends.

On the island where they live, a macabre legend has persisted since ancient times: "A mermaid dwells in a cave deep within the sea of trees, and whoever eats its flesh shall gain eternal life."

However, to the two of them, it was nothing more than a surreal folk tale.

Then one day, after a trivial argument, Guest fell down a flight of stairs when Yuzuriha brushed them off, leaving Guest in a coma and critical condition.

The doctor told him there was little hope for recovery. Facing the heartbeat that grew weaker by the day, Yuzuriha continued to regret his mistake.

Unable to bear the fear of losing his best friend, Yuzuriha reached out to the forbidden mermaid legend of the island as a last resort.

In the middle of the night, Yuzuriha headed secretly to the cave deep in the sea of trees, obtained "that thing," and fed it to the comatose Guest.

A taboo committed to save a life. The price for it will drastically derail the fates of these two—.