Sacha, a traveling Pokémon trainer, enters the crisis threatening the Diamond Domain. He and his companions meet Diancie, the Mythical Pokémon ruler whose domain is endangered by its failing Heart Diamond. Recognizing the threat, Sacha allies with Diancie to seek the Legendary Pokémon Xerneas, believing it holds the power to restore the Diamond Domain's energy source.

Their journey brings them into conflict with thieves attempting to exploit Diancie's diamond-creating abilities. This clash inadvertently awakens the Legendary Pokémon Yveltal, a force of destruction, from its dormant cocoon. Sacha confronts the resulting rampage, coordinating efforts to protect Diancie and mitigate the widespread threat unleashed by Yveltal.

Throughout the film, Sacha demonstrates his commitment to safeguarding Pokémon and resolving conflict. He supports Diancie not only physically but also in its struggle to unlock latent abilities crucial for creating a new Heart Diamond and ensuring the Carbink's underground home survives. His intervention counters both the human antagonists and the catastrophic potential of the awakened Yveltal.

The resolution sees Sacha contribute to containing the destruction and enabling Diancie to fulfill its role as ruler by harnessing its true power. This restores stability to the Diamond Domain, concluding Sacha's involvement in this adventure.