The Heart Diamond, the sole energy source of the underground kingdom called the Diamond Domain, is failing and needs replacing. The only one who can create a new Heart Diamond and save the kingdom is the Pokémon Diancie, the kingdom's Princess, but she does not yet have the power to make one. Diancie goes on a journey to meet the life-giving Legendary Pokémon Xerneas, who will allow Diancie to discover the power to create a Heart Diamond. After months of searching for Xerneas, Diancie goes to explore a city she spotted. She meets the Pokémon trainer Ash Ketchum and his friends, Pikachu, Serena, Clemont, and Bonnie. Ash and his friends join Diancie on her quest to meet Xerneas.