We are in spring, during the summer vacation before entering high school. A young boy, Ki-Ichi Airu, decided to return to the village of his ancestors,Yatake, to find a strange sword - "Karasumaru" - that belonged to his great-grandfather. En route, the young man gets lost in the mountains. Fortunately for him, Sumiko, a young girl passing by there, leads him to Kebigasawa, a village near Yatake. Ki-Ichi soon notices that the women's behavior is very strange. In the remote village of Kebigasawa, where sexual mores are somewhat unusual... the boy will then be drawn into a spiral of initiation to which he would not have participated, which would attract the ire of many villagers. Our hero, who could flee from the beginning, however, will do anything to get the sword which he came for.

Kebigasawa village, built by a retired former local lord to protect themselves if attacked his castle, was lost in the middle of nowhere. Before the development of modern communications, reproduction of the population was very quickly to a wall for lack of new blood, inbreeding might not have good genetic results. The ancients had understood and had developed cultural stratagems to level the problem. Thus, any woman of 33 years was considered in her "bad year". She would avert this fate by making love with a younger man, preferably from outside. On this basis, a sanctification of the sexual act is in place, with many rites of initiation for younger adults. Paradoxically, this has the consequence of taking away all our societies taboo Judeo-Islamo-Christian or modern Japanese tend to associate with.