Kono Yo no Hate de Koi wo Utau Shoujo YU-NO: Mugen no Heiretsu Sekai

Takuya Arima is a teenage student whose father was a great historian and had great research and discoveries when he suddenly disappeared. After some time in the summer vacation from his school, Takuya received a letter from an unknown source with an envelope. When he opened it and read it about the information that was written by his father. In that letter, his father tells him about how he disappeared and reached a parallel world, He doesn’t take it seriously but later he focuses on it and goes in search of his father because his father always talks about to travel in Parallel universe and that envelop consists of a device that help them to travel. He wanted to know that is his father was alive or not. If yes, then where is he right now? How did he get there?