Kai has moved from Tokyo to a lonely fishing village with his father and his grandfather. One day, his classmates invite him to join their band. As he reluctantly follows them to Merfolk Island, their practice spot, they meet Lu, the mermaid girl. Since ancient times, the people of Hinashi Town have thought that mermaids brought disaster. Something happens that puts a huge rift between Lu and the townspeople. Will Kai's cry from the heart save the town?