Momoko Kuramoto, born prematurely alongside her twin brother Riki, faces both physical and mild mental disabilities stemming from birth complications. These challenges initially prevent her attendance at a regular elementary school, causing daily distress manifesting as morning tantrums. Her family soothes her by singing her cherished melody, "The Song of Frogs."

Momoko longs persistently to be "normal" and join Riki at school. This dream becomes reality when her school launches an integrated education program, enabling her enrollment in Riki's class. Despite her disabilities, she exhibits cheerfulness and fierce determination. During physical activities like gym class, she demonstrates resilience, refusing help after falls and insisting on completing tasks alone. Her presence initially challenges classmates unfamiliar with disability, sparking conflict when a bully named Ryuji mocks her after a dodgeball game. This provokes Riki's protective instincts, leading to a physical fight.

Her relationship with Riki is central. Though she admires him, calling him "Onii-chan" (big brother) despite their twin status, Riki struggles with jealousy over the disproportionate parental attention her health needs demand. These needs include frequent hospitalizations that leave Riki feeling neglected. Momoko's integration fosters gradual peer acceptance. Classmates, motivated to lift her spirits during a later hospitalization caused by her worsening condition, resolve to win a relay race to share positive news. Momoko inspires this collective effort, even as her health continues its decline.

Her condition, established early as life-limiting though never explicitly named, ultimately proves fatal. She dies in the hospital, smiling peacefully surrounded by her family and classmates. Her death profoundly impacts those around her, acting as a catalyst for personal growth. One year later, Riki and his friends actively support individuals with disabilities, crediting Momoko for fostering their maturity and compassion.