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

Takuya Arima is a young student whose father, a historian who has conducted various researches, disappeared recently. During a summer vacation Takuya discovers a strange orb in his father's room, along with a letter containing information about the existence of a parallel world. At first Takuya doesn't take it seriously, but soon he realizes that he possesses a device that allows him to travel to an alternate universe. Is his father alive, after all? If so, where is he? Meetings and conversations with many beautiful women, as well as a suspenseful investigation await Takuya!