Episode 26.5, reward for reaching at least 1000 sales for the first Blu-ray volume. The episodes shows different possible outcomes for the protagonist. 

Takuya Arima is a young student whose father, a historian who has conducted various researches, 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?<br><br>

(Source: TLWiki)

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