The year is 2026, October 31st. Modern Japan is in the middle of Halloween. It was peaceful until the Great Hakurei Barrier and Yukari Yakumo's barriers vanished, scattering Gensokyo's residents across Japan. Among the residents, some are in chaos, some are calm, and some are trying to restore order. Cunning youkai are eating humans or destroying buildings, while others try to stop them.

The outside world is in turmoil due to the sudden appearance of non-humans. In Akihabara, which was bustling with Halloween, people mistook youkai for cosplayers. Those who joked about the "high quality costumes" were sometimes eaten and killed by youkai driven by the urge to consume humans. Touhou Project fans praised the "high quality Touhou cosplays," leaving the actual residents confused about what a "Touhou character" even is. Areas with casualties fell into panic (though some stayed in denial, thinking it was a movie). Akihabara is just one example; this is happening everywhere. The general perception is either "Monsters appeared" or "There are Touhou cosplayers."

On the Gensokyo side, those with a sense of justice or those who hate noise, like Reimu Hakurei and Marisa Kirisame, are currently trying to neutralize attacking youkai. Yukari Yakumo is acting calm on the surface but is secretly panicked by this unexpected situation.

Note that there are few youkai originally in the outside world. Any that were already there are weak ones in haunted spots who don't leave their territory. They are generally weak. However, if the "fear of all youkai" increases among humans due to this incident, all youkai will become stronger. This is a general power boost; it doesn't close the gap between youkai, so the original hierarchy of strength remains.