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Kono Yo no Hate de Koi wo Utau Shoujo YU-NO (Visual Novel)

Arima Takuya is a young student whose father, a historian, devoted his life to various kinds of research and has recently disappeared from sight. He was presumed dead by many. During the summer break Takuya receives a mysterious package from his father with a letter attached - he learns about the existence of parallel timelines.