Angry at her parents' decision to divorce, Mao Morishita decides to live with her grandfather in the countryside of Gunma Prefecture.
Meanwhile, Mao's classmate, Chika Nomura, finds an abandoned shiba inu puppy in the river, but because her apartment complex does not allow its tenants to keep pets, she decides to take care of him in secret and names him Dan.
However, Chika has grown so fond of Dan that she walks to Tochigi Prefecture by herself just to visit him, and when Mao realizes the two are inseparable, she convinces Saki's uncle to let them take him back.
Mao and Yayoi persuade Tsuneo to let them keep Dan at the apartment complex, and he finally relents, adopting him.
When she and her father leave on the bus as they move out, Mao sees all of the friends she made at Gunma Prefecture sending her off, and she says goodbye to them.
Koinu Dan no Monogatari: Original Soundtrack (仔犬ダンの物語 オリジナルサウンドトラック) was released February 14, 2003 under the Zetima label.