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  The setting is modern-day Japan. In the world of humans, countries are recognized merely as "nations." However, in reality, each country possesses a being known as a Countryhuman, who lives carrying the history, culture, and memories of their people.

They are fundamentally forbidden from interfering in human society.

However—in Japan, there exists a "Boundary" where these national entities gather in secret.

Located deep beneath Tokyo, it is a place unrecorded on any map.

There, nations conduct diplomacy, exchange information, and occasionally handle incidents that must remain hidden from humanity.

Then, one night.

Japan suddenly vanishes from a portion of everyone's memories.

Japan enters the living room where everyone is, as usual.