People born with unusual appearances and powers, degraded as "unhumans," are heavily discriminated against in the Kingdom of Blognig. However, unknown to the people, their own princess has been hidden away because she was born as one of these so-called monsters—a mermaid!

Princess Mio Quartzlis Tenerwalt is betrothed to Prince Chika of the neighboring kingdom, Aquatia. With only six months before her engagement on her 16th birthday, Mio's father hires a young man to teach his daughter Aquatian customs, 17-year-old Yuri Ban, who is unlike anyone Mio has ever met.

Besides being the only outsider to learn of Mio's true form, Yuri is also the only one who treats her with pure kindness, even welcoming her to be proud of who she truly is. From Yuri's tutoring, Mio has gained more than just royal etiquette—the confidence to make a step toward her truth, as well as against what she, and everyone else, was raised to believe.