Ms. Rieko Sakamoto teaches music at a Kiba, Tokyo elementary school in 1956, approximately a decade after World War II. As the school's new music teacher, she demonstrates strong dedication to her role. Her passion for music education stems from a desire to honor her older brother, Masaki Sakamoto, an airforce pilot who disappeared during the war. Masaki, seven years her senior, acted as her moral mentor before his death.

Ms. Sakamoto initiates and leads the school's choir program, aiming for competition in a regional chorus tournament. She views this effort as a way to cultivate shared pride and cultural continuity within the school and wider community. She considers traditional Japanese children's songs (Doyo) fundamental to Japanese cultural identity, especially during Japan's transition from post-war poverty and its reintegration into the international community.

Her leadership encounters challenges when the choir's tournament participation is canceled due to misconduct by key students, Akira Yanagisawa and Gonji Abe. Following this setback and the accidental death of Shizu Miyanaga, a transfer student aspiring to be a singer, Ms. Sakamoto perseveres. She transforms her grief and dedication into renewed efforts after the choir's reinstatement. Her commitment extends beyond musical instruction; she fosters emotional resilience and collective purpose among her students, helping them process loss while preparing for the competition.

Her journey moves from personal mourning to communal stewardship, emphasizing cultural preservation as a form of healing and hope. The choir's efforts culminate as Japan joins the United Nations, symbolizing a new national era.