[Either a world without Angels, or a world where the Lilim haven't yet made contact. Interesting.
Now, which story to give her - the government story, or the truth? He'll start with the official story, and see if she gives him reason to elaborate.]
Second Impact was a meteor strike in Antarctica. It immediately melted the polar ice caps and submerged most of the world's landmass. Between that and the wars that followed, the world's population was halved. [Still talking in that calm, pleasant tone.] Tokyo was nuked, so they built Tokyo-2 as the seat of government where Matsumoto City used to be, and Tokyo-3 over Hakone.