Hasukawa Kazuhito got amnesia so he only remembers the last 5 years of his life. His girlfriend is Aihara Hikaru, who always gets into trouble in her work. One day he receives a letter and meets the girl who said she is Kazu girlfriend before he amnesia.
Still confused, Kazuhito is to be sent to future by Makoto, a empoyee of the Culture Department. She tells Kazuhito that his real indentity is Kai, the Legendary member of Nightmare, a illegal organization under the Culture Department.
Kazuhito must adapt to his new life. Tokyo was separated from Japan, and he knows nothing about the suspicious organization with young members. And who was Kai?