Yoko Nakajima, an unhappy high school student, is one day suddenly faced with a strange man who swears allegiance to her. After a battle with demon-like beasts, he then takes her to another world along with two of her classmates. There, her appearance has changed and she can understand the language even though her classmates cannot. But their status as "Kaikyaku" (people who come from Earth) makes them hunted fugitives, so they wander the land of the 12 countries, simply trying to survive and to figure out the reason why they were brought to this world.