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The movie is about AU Ash starting his adventure and Necrozma fucking with him until Ho-Oh appears to set things right.
Pretty much Ash is "Linked" to Ho-Oh and the reason it never appeared in the Anime timeline was because it did it's job by granting Ash an "Joy filled and Adventurous life", but when an Ultra wormhole opens and releases Necrozma, it fucks the timeline up, causing Ash to miss out on Key points in his early adventure, forcing Ho-Oh to intervene.
This movie is always supposed to crossover into the Anime in someway.
