Fumi Koito used to be raised by an old housekeeper named Hatoko Natsuyaki. He would call her "Peppermint Nanny" because she would eat his leftover peppermint candy drops—because he hates how they taste—then breathe into his nose because he likes the smell.

Now Fumi is in his mid-20's and living on his own as a cram school teacher. One day a little girl no more than nine years old shows up at his house. It turns out she is his old nanny who misses cooking and tending to his every need. But because she is pushing eighty and has grown too frail, she drank a mysterious bottle of medicine to regain her youth—and accidentally drank too much.

Thus begins their new life together, with the reader frequently left guessing who is taking care of whom.