Takuya Arima is a young student whose father, a historian who has conducted various researches, disappeared recently. During a summer vacation Takuya discovers a strange orb in his father's room, along with a letter containing information about the existence of a parallel world. At first Takuya doesn't take it seriously, but soon he realizes that he possesses a device that allows him to travel to an alternate universe. Is his father alive, after all? If so, where is he?