‘Onha’ has never once stepped foot outside the palace. Though she’s always been curious about matters of sex, she’s baffled when told that, as an adult woman, she must marry a child who’s not even seven...

On the day she turns 20, Hiyori Izumo, who lives in a poor village, marries Saku, the head of the Minazuki family, who is seven years older than her, according to the village tradition that "children with a sun...