Nanaka Kirisato is a studious, goal-oriented high schooler at 17 years old. She frequently scolds her friend Nenji Nagihara, who she sees as immature because he fights other boys. However, their dynamic changes when she falls down a flight of stairs during an argument with him, causing a brain injury that regresses her to a six-year-old mindset. Nanaka's father and Nenji keep her condition under wraps as she navigates the challenges of growing up all over again while trying to lead a normal life.