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. (thetvdb.com)