Kei Takanashi is a young, not-so-successful screenwriter. One day, she finds an unexpected visitor at the door of her apartment. A little girl named Niko Kobako, a secret child of a famous actress Mifuyu Shibano. Niko is sent to Kei because she is the only person who knows this secret, since she used to work for Mifuyu when she was not so famous. As she begins to live with Niko, she finds the girl is very self-contained. She tries an 'exchange diary' to communicate with her.