Kazuhito Hasukawa suffers from amnesia, recollecting only the last five years of his life. His significant other, Aihara Hikaru, tends to find herself in circumstances that get her into trouble with her job. One day, a letter arrives, informing Kazuhito that he had a girlfriend named Kazu prior to losing his memory.
Makoto, an employee of the Culture Department, confuses Kazuhito even further by informing him that he is to be sent to the future. She discloses that Kazuhito's true identity is Kai, a legendary member of Nightmare, a renegade organization that operates within the Culture Department.
Kazuhito must adjust to this unfamiliar existence. Tokyo has become an independent entity separate from Japan, and Kazuhito is oblivious to the questionable organization's young members. Furthermore, who exactly is Kai?
