Each day, a young girl in a mask stands by the shore and sings a nostalgic tune. After enduring two sudden, heart-wrenching partings in childhood, Nino Arisugawa has been singing to the ocean, bound by a promise made with her two childhood friends: her first love, Momo Sakaki, and a boy who composed music, Kanade "Yuzu" Yuzuriha. Having never met each other, the boys both individually promised that if Nino was ever separated from them, her voice would serve as the beacon to reunite them once again. After six long years, destiny has finally placed Nino, Momo, and Yuzu in the same high school. However, the passage of time has altered many aspects of their lives—while Nino relentlessly attempts to fulfill her childhood promise with the boys, Yuzu's feelings for her from the past resurface, and Momo goes to great lengths to prevent a reunion with Nino. Through music, will they be able to mend their friendship and overcome all the feelings involved in this complicated love triangle?