Historia
This is the 8th Movie. In the town of Lohta, a festival is thrown praising "Aaron the Wave Leader Hero" who, according to the legend passed down through the town's generations, stopped a war centuries ago by using the power of "the Tree of the World's Origin." Satoshi and Pikachu participate in a battle to decide this year's hero and after their victory, they are chosen as the "Wave Leader Hero." A grand dance party is held in the castle to celebrate the victor, but suddenly the phantom Pokemon Mew appears and takes Pikachu.
Sinopsis vía AniList (en inglés).