Takuya Arima's father — a historian with a reputation for unconventional research — has vanished. The disappearance is unexplained and the investigation has gone nowhere. During summer vacation, Takuya finds a strange orb in his father's study alongside a letter describing, in specific detail, the existence of a parallel world. His first instinct is skepticism. Then he realizes he is holding a device that can do exactly what the letter describes. His father's research was not theoretical. If his father is alive, the answer to where he went is somewhere the device can reach.