Ark Academy, chosen as a model case for schools that integrate different races. The new transfer student was Elio, a member of a hugely popular mermaid idol group. With her lovely appearance and shy demeanor, Elio was loved by her classmates and quickly made friends. However, she harbored a certain problem. Due to the characteristics of her mermaid species, Elio's body would soon transform into that of a man. "I...I transferred here to get the coach's help..." Her developing body. Her breasts just beginning to swell and her waist slender. With innocent eyes, Elio urges him to have sex. The homeroom teacher is torn between lust and morality, but there's no way he can refuse... The mermaid's "song" permeates his consciousness. A mermaid idol surpassing the popularity of AKB48 and TWICE is born!! With a mermaid's uniquely clear singing voice, your baton should also go fortissimo!