The setting of the tale takes place in Diamond Ore Country, where Princess Diancie reigns. This underground land is inhabited by Carbink Pokémon who coexist peacefully. The Sacred Diamond serves as the main energy source for the country, but only the princess has the power to create it. Unfortunately, Diancie lacks the necessary ability to produce the crystal, and the existing diamond's power is dwindling rapidly, putting the country in peril. In search of a solution, Diancie joins forces with Ash and Pikachu to track down the divine Pokémon Xerneas, known for its sacred power. However, their quest takes an unexpected turn when they come across the cocoon of Yveltal, a notorious creature that had once brought destruction to Kalos.