With the power of science inhabitants of the Earth planet went off to colonize other planets. Only Nihon nation had to remain on the planet because of catastrophic recession, so the star was renamed to Nihon.
New civilizations often look back to the Nihon star as source of ancient culture and traditions, so Nihon starts to export its culture in exchange of trade benefits. Among the other offerings Nihon provides specially trained prostitutes.
Main character is a young man who is entrusted to teach new prostitutes special techniques and refine their skills.

[From vndbreview]