One morning during summer vacation, Natsuki and her friends gather in front of Takasaki Station and set off for a beach in Niigata Prefecture. During the journey, Natsuki and Miori's classmate, Honjo Serika, bond over their shared love of music. Despite being swept up in Serika's unique pace, Natsuki's circle of friends expands. Upon arriving at the seaside town, the group enjoys swimming, a BBQ, and fireworks. It's the sparkling summer day that Natsuki had always dreamed of.

With all the band members finally assembled, Natsuki and the others begin practicing in earnest. For their first song to perform at the school festival, they decide to jam to a song written and composed by Serika. During the performance, Natsuki feels the stark difference in skill between herself and the other members and loses confidence. However, Serika encourages her, saying she likes Natsuki's singing voice. A week after the band was formed, Serika, struggling to write lyrics for their second song, which is themed around "the past" and "regret," asks Natsuki to write the lyrics for her.