Kai, a young middle schooler, lives in Hinashi Town, a lonely fishing village, with his father and his grandfather, a sun-umbrella maker. He used to live in Tokyo, but after his parents divorced he moved back to his parent's home town. Kai has trouble telling his parents the complicated feelings he has for them, and he's lonely and pessimistic about his school life. One of his joys is uploading songs he writes to the internet.

One day, his classmates Kunio and Yuho invite him to join their band, "SEIRÈN". As he reluctantly follows them to Merfolk Island, their practice spot, they meet Lu, the mermaid girl. Lu sings merrily and dances innocently. As Kai begins to spend time with her, he starts to be able to say what it is that he's really thinking.

But 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. And then, the town is in danger. Will Kai's cry for the heart be able to save the town?

Type: Movie
Episodes: 1
Status: Finished Airing
Aired: May 19, 2017
Producers: Fuji TV, Toho, BS Fuji
Licensors: None found, add some
Studios: Science SARU
Source: Original
Genres: Adventure, Music, Fantasy
Duration: Unknown
Rating: None

Director: Masaaki Yuasa
Script: Reiko Yoshida (Aria The Animation), Masaaki Yuasa
Producer: EunYoung Choi (Ping Pong The Animation Assistant Director)
Original Character Design: Yoko Nemu (Gozen 3-ji no Muhouchitai Mangaka)
Animation Director, Character Design: Nobutake Itou (Kaiba)
Art Director: Kouji Oono (Ookami Kodomo no Ame to Yuki)
Director of Photography: Batisto Perón
Flash Animation: Abel Góngora, Juan Manuel Laguna (Garo: Honoo no Kokuin Animation Director and Photography)
Sound Director: Eriko Kimura (Yojouhan Shinwa Taikei)
Music Composition: Takatsugu Muramatsu (Omoide no Marnie)
Studio: Science SARU

Lu: Kanon Tani (Kimi no Na wa.)
Kai: Shouta Shimoda (Live-action Kuchibiru ni Uta wo)
Lu's Father: Shinichi Shinohara (Video Game Yakuza: Kiwami)
Comic Duo: Chidori
Grandfather: Akira Emoto (Furusato Saisei: Nihon no Mukashibanashi)
Kunio: Soma Saito (Zankyou no Terror)
Yuuho: Minako Kotobuki (Hibike! Euphonium)