In a country called "Diamond Domain" (ダイヤモンド鉱国 Daiyamondo Kōkoku?) lies the powerful Heart Diamond (聖なるダイヤ Seinaru Daiya?, "Holy Diamond"). The Heart Diamond served as the kingdom's source of energy. It also maintained the Ore Country for centuries. Many Carbink live in the Diamond Domain. This includes their princess known as the Jewel Pokémon named Diancie. Diancie created the Heart Diamond. Diancie no longer has the power to control the Heart Diamond. Diancie's country is falling into chaos as a result of losing control of the Heart Diamond.
One day, Diancie meets with Ash and his friends. Diancie asks them to go on a journey to find the Life Pokémon named Xerneas in order to restore the Heart Diamond. Standing in their way is the Pokémon of Destruction named Yveltal who once stole all of the life in the Kalos Region. During their journey, they are attacked by Team Rocket and the jewel thieves named Marilyn Flame and Ninja Riot. Diancie and Ash and his friends are aided by Millis Steel and her father named Argus. Millis and Argus Steel are thieves that want to use Diancie's power. They also encounter the Cocoon Of Destruction who is known as Yveltal.