One New Year’s Day when narrator Ruru is in her mid-twenties, her 80-year-old grandmother Toyose learns that Ruru has gotten a story published in a literary journal, and urges her to write about the major family secret she has related to Ruru in bits and pieces over the years. Toyose collapses that same day, and passes away two days later.
The secret has to do with Toyose’s twin sister Kaei. The twins were born into a prominent family in Nihonbashi, Tokyo, near the end of the Meiji period (1868?1912). Toyose thrives and grows normally, but the family worries that Kaei’s development appears to be delayed. As time goes on, however, they realize that she is in fact reaching all of the standard milestones, only at a far slower pace. When Toyose has matured into a young woman of marriageable age, Kaei is still a little girl. Because Kaei is obviously abnormal compared to her twin sister, the family moves to a newly built estate in the suburbs where she can be kept out of sight, and she is subsequently sent to seek treatment in England accompanied by her attending physician.
Fourteen years later, in 1944, Kaei returns to Japan from Manchuria, where she had gone after England. Toyose is by this time middle-aged, past her child-rearing years, but Kaei is a beautiful young woman in her prime. For a time she lives as if under confinement in the suburban residence the family had built to keep her out of sight, but a short while after the end of World War II, she moves out to live on her own.
Ruru’s mother (Toyose’s daughter) dislikes Kaei, who still comes to visit the family home from time to time. Then Kaei’s visits stop. Ruru’s mother does not tell her daughter about Kaei’s secret, but she repeatedly finds occasion to note that it’s best for people to age and die normally. When Kaei comes to Toyose’s funeral, the woman who should be just as old as Toyose appears to be no older than the last time Ruru saw her. Ruru’s narrative then comes to an end with a scene in which she is talking with Kaei in the family’s backyard, where the geraniums are in bloom.
Several decades go by. Ruru’s parents as well as Ruru herself are long dead, and even Ruru’s son and daughter are getting on in years, but Kaei lives on. She picks up the story where Ruru’s narrative had left off, revealing secrets that Ruru herself had no way of knowing, such as the fact that she had had an affair with one of the men Ruru was seeing. Her tale brings into relief the loneliness she feels as her life crawls on at its much slower pace, watching people born long after herself living full lifespans and preceding her in death one after another.