On a night with a full moon, Subaru, who was a merman, coincidentally rescued a man who had sunk into the ocean. As a result, he decided to trade his tail for a pair of legs to meet Tatsuki, the man he had saved. Despite the pain in his feet and the loss of his voice, Subaru was content to be near Tatsuki. However, his happiness was short-lived as he discovered the reason behind Tatsuki's presence in the ocean that day. A one-shot entitled "Undying Illness" was included in the doujinshi sequel to "Mister Mermaid."
