Nanaka Kirisato, a diligent high school student, holds firm ambitions for her future. She often chides her friend Nenji Nagihara for his unruly behavior, involving himself in brawls with other boys. However, their dynamic shifts dramatically when a quarrel leads to Nanaka falling down stairs, causing a brain injury that regresses her mental age to six. Concealing this setback from others, Nanaka's father and Nenji navigate the challenges of aiding her to relearn life's basics and mature afresh.