A girl who spent seventeen years being serious about everything gets reset to six, and suddenly the delinquent she was always criticizing becomes the only person who understands what she's actually lost. The show builds its comedy on a brutal premise: she's happier now, and everyone knows it, and nobody can say that out loud. Watching Nenji protect her secret while she forgets why she ever needed protecting becomes something sadder than any amnesia plot has the right to be.

Seventeen-year-old Nanaka Kirisato takes her schoolwork and her future seriously, and she often scolds her best friend Nenji Nagihara for acting like an immature troublemaker who keeps getting into fights with other boys. Everything changes when, after a heated argument with Nenji, she tumbles down a flight of stairs and suffers a brain injury that leaves her with the mind of a six-year-old. Nenji and her father now have to hide what happened while Nanaka tries to live an ordinary life and do her growing up a second time. The story mixes comedy, drama and romance in an everyday slice of life setting.