Takuya Arima is a young student whose father, a historian who has conducted various investigations, disappeared recently. During a summer vacation, Takuya receives a peculiar package from his missing father, along with a letter containing information about the existence of various parallel worlds. At first, Takuya doesn't take it seriously, but soon he realizes that he possesses a device that allows him to travel to alternate dimensions. Is his father alive, after all? If so, where is he? ([Source: VNDB](https://vndb.org/v1377))