The only thing that can be said with certainty is that Oba-san and the two schoolgirls had never met before their paths converged that afternoon. There was, as well, a very high probability that their brief encounter would be the only meeting of their lives.

In a city the size of Tokyo, it had been sheer chance that propelled the silver-haired woman to that particular seat in that particular carriage on that particular Ginza Line train at that particular hour. And it had also been mere happenstance that brought the little girls to the same carriage of the same train at the same time.

Only two seats were empty when the children — aged 10, I guessed, and clearly classmates just out of school — stepped on board. Although they no doubt would have preferred to sit together, neither hesitated for a moment before claiming the only remaining vacancies in the carriage. One seat was across the center aisle from Oba-san, but the other was right next to her.