Kono Yo no Hate de Koi wo Utau Shoujo YU-NO

Takuya Arima's father, a historian and researcher, disappears under mysterious circumstances and later sends his son a package containing a letter about the existence of parallel worlds. Inside is an unusual device that allows Takuya to move between different branches of reality and return to earlier points while retaining what he has learned.

He begins using the device to search for his father, only to discover that people close to him are hiding information connected to the same research and that events do not always unfold identically from one world to another. Every jump opens one possibility and closes another, forcing Takuya to assemble the truth across parallel lives. Finding his father first requires understanding why he left behind a key capable of crossing time and dimensions.