Abyss Azure no Zainin

Beneath the ocean's surface, a hidden nation exists, largely unknown to humans. Mermaid Jo lives there in quiet contentment until her friend Ryuu is imprisoned for violating a serious law: revealing the existence of the underwater world to humans. The story follows Jo as she embarks on a journey to uncover why Ryuu betrayed the secret and who she told, in hopes of saving her friend. Along the way, Jo's adventure leads to the development of her feelings.