Ark Academy was chosen as a model case for a school with different races. The new transfer student is Erio, a member of a popular mermaid idol group. Erio's pretty appearance and shy demeanor make her endearing to his classmates, and she soon finds a rapport with them. However, she has a certain problem. Erio is a mermaid, a characteristic of her race that will soon cause her body to turn into a man. "I... transferred in to help Coach..." The developing body. Her breasts are almost swollen and she has a slender waist. Erio presses with eyes that still have an innocent look in them. He is a homeroom teacher torn between sexual desire and ethics, but there is no way he can say no... The mermaid's song seeps into his consciousness. The birth of a mermaid idol that surpasses the popularity of A○B 48 and TWI○E! Your tact is also fortissimo with the clear singing voice peculiar to mermaids!