30-year-old bachelor businessman Daikichi Kawachi encounters Rin, a girl he's never seen before, at his grandfather's funeral. Surprisingly, the 6-year-old girl turns out to be his grandfather's love child. Daikichi doesn't like children, but the cold attitude of his relatives towards Rin makes him uncomfortable. And then, the relatives' meeting to decide who will have custody of Rin begins at last...

What kind of clothes to buy her? What to feed her? What to do about nursery school? Daikichi's new life raising a 6-year-old girl is an uninterrupted stream of bewilderment. Rin looks anxious on her first day of nursery school, so Daikichi does a "pinky swear" with her, promising to pick her up as soon as he can. And Daikichi, who has no confidence he can live with Rin for long, starts thinking about meeting Rin's mother...

Daikichi goes back to his parents’ house with Rin in tow in order to search for clues about her mother. At first, his parents were opposed to him taking custody of the girl, but now they’re not entirely dissatisfied about having what amounts to a grandchild. And at his grandfather's house, Daikichi discovers a mystery modem. This leads him to think about his life with Rin and he makes an important decision.

Rin befriends Kouki, a boy she met at the local nursery school, while Daikichi's pulse races a bit when he meets Kouki's beautiful single mother. Later, the modem Daikichi found leads him to his grandfather's last words. And from there, he receives a contact number for Rin's mother, Masako...

Daikichi finally meets Rin's mother, Masako, and he's shocked by her frankness. When they part, Masako asks Daikichi for a favor, and it's an issue that Daikichi can't avoid, as long as he's going to raise Rin. Daikichi tries to resolve it by having a heart-to-heart conversation with Rin, but he gets an unexpected answer...