Kai Ashimoto's grandfather strengthens the belief in the legend of mermaids that lurk close to Merfolk Island, where they prey on humans in Hinashi Town. This tale has led to the Ashimoto household rarely discussing music since it is believed to attract the merfolk, who were responsible for the death of Kai's grandfather's mother. Nevertheless, Kai finds comfort in producing looped beats on his laptop. Kunio and Yuuho Ebina, Kai's schoolmates, invite him to join their rock band after coming across one of his videos. Kai seems apathetic at first but eventually agrees to accompany them to Merfolk Island for a rehearsal session. While there, Kai witnesses something inexplicable. Later that night, Kai discovers a mermaid that appears familiar, drawn to her as she moves and grins in time with his beats. As Kai and this mermaid, Lu, spend more time together, he learns that she possesses remarkable water manipulation skills, can metamorphose into a human with the assistance of music, and is desperate to be accepted by people. Kai and Lu's relationship helps to break ingrained boundaries that previously silenced the enchanting melody among Hinashi Town's inhabitants.