Japan. It took 100 years after the shift of the lithospheric plates, which caused flooding in many countries. In order to save the rest of the state, the new cities on water-related sea routes were built.There was a need for staff that monitors the safety of the sea, and the woman quickly occupied this niche in employment. Thus it was created "Blue Mermaid", an elite organization responsible for maintaining the security of the sea, as well as the awareness of customs and cross-cultural and inter-ethnic relations.Childhood friends Misaki Akeno and Tina MOEK dream to join the Blue Mermaid, which the girl enters the marine high school Ёkosuki. Their path to a dream start.