Long, long ago, in an era before Poké Balls existed, a great war broke out among Pokémon. The warrior Aaron and his Pokémon Lucario, striving to stop the war, used the power of Aura to quell the conflict, and people thus hailed Aaron as the "Aura Guardian." A century later, Ash and Pikachu, along with their companions, arrive at the Kingdom of Rota to participate in the Pokémon battles and festival held in honor of the warrior Aaron. Ash emerges victorious in the battles and is bestowed the title of "Aura Guardian," earning the right to wield the "Staff of the Hero" and embody the warrior during the festival. Just as the grand celebration is underway, the Pokémon Lucario, sealed within the "Staff of the Hero," awakens right before Ash's eyes and reveals the secret of Aaron abandoning the castle and fleeing a hundred years ago!? Meanwhile, the mysterious Pokémon "Mew" suddenly appears and takes away Pikachu and Meowth. In search of Pikachu, Ash, guided by Lucario, heads toward the "Tree of Beginning." What mysteries lie hidden beneath the enigmatic "Tree of Beginning"? Where has Mew taken Pikachu? What is the secret behind the legendary warrior Aaron and Lucario's "Aura legend"?