When Japan’s top Pro Heroes are summoned to a classified Hero Commission facility, they’re expecting a national emergency—not a mermaid. Hidden inside a massive tank beneath the Commission headquarters is YN, a mysterious girl from a world humanity never knew existed. As trust slowly forms between her and the heroes assigned to watch over her, long-buried secrets begin to rise from the depths. But with governments, scientists, and powerful organizations closing in, YN’s arrival may change the future of both their worlds forever.