In the underground Diamond Domain, where many Carbink live, the Mythical Pokemon Diancie serves as ruler. The Heart Diamond that sustains the land is beginning to fall apart, and Diancie is not yet strong enough to create a new one. While seeking help from the Legendary Pokemon Xerneas, Diancie encounters a group of thieves who want to take control of its diamond producing power and who awaken the Legendary Pokemon Yveltal from its cocoon in the process!

Can Ash and his friends help Diancie discover its true power, stop Yveltal's rampage, and save the Diamond Domain?