While their mother goes on an assignment to Kyushu, she leaves Naoki and almost three year old Yuuko with their grandparents in the little castle town of Hanaura. Hanaura is in western Japan, on the coast of the Seto Inland Sea, a location that will become relevant later in the novel. The family sometimes call Yuuko Ida, a nickname that Naoki gave her from a Hans Christian Andersen fairy tale.
On the first night in this grandparents' house, Naoki sneaks out to explore.
Naoki did not know how long he had been standing next to the castle moat. His attention was suddenly wakened as he heard the clatter, clatter sound of someone passing by near his feet. At the same time he heard a low murmur, "Gone, gone, can't find her ....., gone."
Although the voice was low and hoarse, it could be heard from down by his feet. Shocked Naoki looked around below him. It was a chair. It was a small - yes, about the size that would just fit Yuuko if she sat down in it - backed, round wooden chair. The chair was walking, clatter, clatter, along the white path at the edge of the the moat, dragging its legs with each step.
The next day Naoki discovers an abandoned house in the woods, and in it the chair from the night before. When Yuuko visits the house, she seems strangely at home there; and the chair thinks that it recognises her as the Ida it knew. But whoever that Ida was must have vanished long ago. Angry at the chair's claims on his sister, Naoki tries to find out what had happened to the people living in the house; but he also starts to wonder whether his Yuuko might be the reincarnation of the Ida that the chair knew.
A visit to Hiroshima suggests what may have happened to the family. A young woman from the town takes Naoki with her to a memorial ceremony for the victims; and Naoki learns about the events that he had only vaguely heard of before.
In this way the fantasy element of the book winds into a story of the atom bomb. The two fit together a little oddly. The chair's sentience is not really motivated; but its character can be seen as a way of approaching the feeling of being unable be come to terms a loss of this kind.
[UPDATE: I forgot to check the Japan Foundation's Translation Database before writing this review. It turns out there is an English translation, Paula Bush, Two Little Girls Called Iida, Kodanasha International, 1985.]