The protagonist, Takuya Arima, is a high school student who has nothing to do. His father is a historian, but he disappeared during his research, leaving him and his stepmother to depend on each other. During the summer vacation, the protagonist received a package from his father, which contained a compass and three crystal beads, as well as a letter saying "Time is reversible, history is irreversible" and some other concepts about parallel worlds, and told the protagonist to go to the famous local landmark Cape Kennosaki to meet someone at night. Takuya arrived at Cape Kennosaki as scheduled at night and found that his stepmother Ayumi and his father's research partner Ryuzoji Kozo were present. The originally kind Ryuzoji suddenly turned hostile and asked Arima to hand over the package he received, and pulled out a gun to threaten him. At the critical moment, a lightning bolt fell and struck the sacred stone of the shrine, and Takuya fainted. When Takuya woke up, there was no one around him. After going home and sleeping, he found something wrong. His stepmother and Longzang Temple had no recollection of what happened last night. Only then did he discover that the relic his father sent him was a magical prop that allowed people to travel freely between parallel worlds. So he launched a series of investigations to find his father's true whereabouts.