Nino Arisugawa sings a nostalgic melody by the beach every day, a ritual rooted in a childhood promise made with her friends Momo Sakaki and Kanade "Yuzu" Yuzuriha. Having been separated from them for six years, Nino is now in the same high school as the boys, but their lives have changed in significant ways. Yuzu has rekindled his past feelings for Nino, while Momo is determined to prevent a reunion.