In Hinashi Town, a seaside village, Kai Ashimoto's family is haunted by the legend of man-eating mermaids near Merfolk Island. Despite the taboo on music in his home, Kai secretly creates beats online. When classmates Kunio and Yuuho discover his talent, they invite him to join their band. Reluctant at first, Kai eventually joins them for a practice on Merfolk Island, where he witnesses a mysterious event. That same night, a mermaid named Lu appears in Kai's room, showcasing her unique powers and kind nature. As Kai and Lu form a deep bond, they challenge the town's prejudices and strive for acceptance and understanding.