Nanaka Kirisato, a seventeen‑year‑old high‑school student, is devoted to her studies and future goals. She frequently berates her best friend, Nenji Nagihara, for being a childish delinquent who spends his time fighting other boys. After a heated argument with Nenji, Nanaka falls down a flight of stairs and sustains a brain injury that causes her mind to revert to that of a six‑year‑old. To protect her, Nanaka’s father and Nenji conceal the injury as she struggles to live a normal life and grow up once again.