Diancie is the princess of Diamond Ore Country, located deep underground, where the Pokémon Carbink live together in peace. The energy source of this country is a giant diamond called the Sacred Diamond, which can only be created by princess Diancie. However, the current princess does not yet possess the power to create such a diamond, and the current Sacred Diamond's power is soon going to run out, which would result in the end of the country. Diancie meets up with Ash and Pikachu, and they set off to search for the legendary Pokemon Xerneas, which possesses sacred power. On the way, they find the cocoon where the Destruction Pokémon, Yveltal, who was said to have once destroyed life in Kalos, lies in wait.

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Parent story: Pocket Monsters XY
Other: Pikachu, Kore Nan no Kagi?
Prequel: Pocket Monsters XY: Koukoku no Princess Diancie