The background story is a world where both the Soviet Union and the US occupied Japan
When the heroes drive through the presumably irradiated landscape, we can see warning signs in both English
and Russian.Maybe it is a Bad End to The Place Promised in Our Early Days: The Cold War in that part got either hot enough for the use of nukes or one or both of the sides hit a nuclear plant, irradiated the whole place and helped the Japanese building these giant underground habitats as compensation.
