Heureka, also known as Bonnie, joins Ash Ketchum, Serena, and her brother Clemont to assist the mythical Pokémon Diancie. Diancie is pursuing the legendary Pokémon Xerneas to restore the failing Heart Diamond. This diamond sustains Diancie's underground kingdom, the Diamond Domain, home to Diancie and many Carbink, which faces collapse as the diamond's energy diminishes.

During the journey, Heureka and the group confront thieves attempting to exploit Diancie's diamond-creating powers. The thieves accidentally awaken the destructive legendary Pokémon Yveltal from its cocoon. Heureka aids the collective efforts to protect Diancie, counter Yveltal's rampage, and secure the Diamond Domain's survival. This involves helping Diancie realize its true power to fulfill its mission.