Seventeen-year-old Nanaka Kirisato is a high school student who takes her studies and life goals seriously. She often criticizes her best friend, Nenji Nagihara, for acting like a childish delinquent who wastes his time fighting other boys. Then one day, following a heated argument with Nenji, she falls down a flight of stairs and sustains a brain injury, causing her mind to revert to that of a six-year-old. Consequently, Nanaka's father and Nenji must keep her injury a secret as she struggles to live a normal life and grow up all over again.