In the coastal village of Hinashi Town, Kai Ashimoto's grandfather is among the locals who uphold the legend that mermaids lurk near Merfolk Island and devour humans. Because music is said to attract these creatures and allegedly led to the death of the elder's mother, it is rarely discussed in the Ashimoto home. Even so, Kai finds comfort in creating looped beats on his laptop and uploading them online. Despite his reserved and gloomy demeanor, Kai is approached by Kunio and Yuuho Ebina, two cheerful classmates who stumbled upon one of his videos. The pair invite him to join their rock band but encounter apathy. Nevertheless, Kai agrees to accompany them to a practice session on Merfolk Island, where he soon witnesses something he cannot explain. Later that night, while composing music in his room, Kai attracts a familiar entity—a mermaid. As she twirls and grins to the sound of his stereo, Kai discovers several aspects about the being who introduces herself as Lu: she possesses incredible water manipulation powers; she sprouts a pair of functioning legs with the sound of music; but most notably, she is a truly benevolent creature who simply wants people to like her. Fostering a dynamic friendship, Kai and Lu gradually come to express their genuine selves, testing the long-standing boundaries that have deafened the resounding harmony among the people of Hinashi Town.