Kai Ashimoto (voice)
Kai se muda com seu pai de Tóquio para Hinashi, uma cidade pesqueira em declinio, para viver com seu avô após o divórcio de seus pais. Kai é incapaz expressar seus complexos sentimentos sobre seus pais, assim ele passa sua vida escolar no ensino médio de forma melancólica. Sua única alegria é fazer o upload de suas composições musicais para a internet. Um dia, seus colegas Kunio e Yuuho convidá-o para se juntar à sua banda, e quando ele relutantemente os acompanha para praticar na Ilha da Sereia, os três encontram uma sereia chamada Lu. Através de seu alegre canto e dança, Kai lentamente é capaz de se abrir sobre seus sentimentos. No entanto, uma calamidade chega e Kai deve gritar do fundo do seu coração, para salvar a cidade.
Sasaki Shohei
Sasaki Ryohei (Yamada Ryosuke), the second son of the Sasaki family, is a high school student who lives in Shimane. He has just been chosen to be the vice captain of the youth soccer club. Setting his sights on participating in the prefectural championship, Ryohei practices every day with his buddies. One day, Ryohei has a severe headache and is brought to the hospital. Ryohei is diagnosed with an acute brain tumor from the test results. The family is told by the doctor that Ryohei will not live long. However, Ryohei musters the strength to say, "I will win. I will beat the illness. " Given a push by his words, the Sasakis try to stay positive.
Satoru
Yuri Kashiwaga was once known as a genius pianist. One day, she returns to her hometown in Goto Islands from Tokyo. There, she begins to work as a temporary teacher at a middle school and advisor for the school chorus. The chorus aims to take part at a competition. Yuri gives the members of the chorus members an assignment. She instructs them to write a letter with the title of “To myself 15 years later.” The 15-year-old boys and girls write about their secrets and worries.
