When Pikachu is taken to the Tree of Beginnings by the playful Mew, Ash Ketchum and friends are guided to the tree by Lucario, a time-displaced Pokémon who seeks answers regarding the betrayal of his master.

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. Soon afterwards, the Wave Leader Pokemon "Lucario" is freed from his centuries-long seal and meets Satoshi. Satoshi must solve the various mysteries of where Pikachu is, what Mew's after, what the secret of the legend of the Wave Leader is and how it ties in with Lucario's hidden past as he makes his way to "the Tree of the World's Origin" with Lucario.

The world of Pokémon has many legends, but the people of Cameron Palace believe in one very special tale: the legend of the Aura Guardian. Long ago a hero and his Pokémon, Lucario, saved the world from destruction- but can it be true? Ash and his friends find out for themselves when Lucario is awakened and Pikachu disappears! With Lucario's help, Ash sets out on a journey to find Pikachu, a journey that leads him to a mystical tree and the legendary Pokémon, Mew! The future of a world and the fate of Ash and Pikachu will depend on the power of friendship between humans and Pokémon.