100 years ago, the desert city of Diesel was severely damaged by a battle between Pokémon. The battle was caused by the existence of a Pokémon. However, this Pokémon was later sealed by someone, and now the city has regained its former peace. During the journey of Xiaozhi and Pikachu, they met the phantom Pokémon Hoopa, who has a light ring that can summon everything, and the girl Maiari who takes care of it. Hoopa, who likes to play pranks, uses the mysterious ring to take out the favorite donuts and various things to surprise everyone, and he enjoys it. The innocent Hoopa calls Xiaozhi and Pikachu Xiao~Zhizhi and Pi~KaKa. Hoopa, who has become friends, embarks on a journey with Xiaozhi and his group. As soon as they arrived in Diesel City, they met Maiari's brother Balza, but Balza looked strange. A closer look revealed that he was holding the prop prison bottle that sealed a Pokémon that once destroyed the city! An evil breath overflowed from the bottle and became a huge shadow. The shadow gradually reveals its true form, which is Hoopa's original separated form, Ultra Hoopa! No one could have imagined that Ultra Hoopa was the culprit of the Pokémon riot 100 years ago. The bottle seals Hoopa's original power. That powerful power gradually turns into anger over the long years, and finally condenses into a shadow and revives in the form of Ultra Hoopa! The liberated Ultra Hoopa begins to run wild, summoning legendary Pokémon from the 6 rings: Proto-Grawdon, Proto-Kyogre, Diorga, Palkia, Giratina, and Kyurem! If this continues, the city is in danger. In order to protect the city, Hoopa uses the rings to invite new legendary Pokémon! It's a true "Legend vs. Legend"! This battle caused the earth to crack, the sea to separate, and space to become distorted! Ash and his friends fight together with the legendary Pokémon invited by Hoopa. Can they protect this Diesel City! ? And how will the battle between Hoopa and Ultra Hoopa end! ? The most epic showdown in the history of Pokémon movies will unfold here!