Suzumiya Haruhi no Shoushitsu

Also known as:  The Disappearance of Haruhi Suzumiya, Suzumiya Haruhi Movie

Suzumiya Haruhi no Shoushitsu (The Disappearance of Haruhi Suzumiya) is the first anime film in the franchise, and also the title of the 4th volume in Nagaru Tanigawa and Noizi Ito's original light novel series, which studio Kyoto Animation adapted for this movie. In it, Kyon comes to school to discover reality has altered to deny that Suzumiya Haruhi ever existed. Kyon then has to travel back through time to find Haruhi and correct the oddity.

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Producers, official website and other details: AniDB, ANN, Wiki