Located in the Kalos region, there is a harmonious and peaceful underground kingdom - the Diamond Kingdom. The naughty and lovely elf princess Diancie (voiced by Marika Matsumoto) is much loved, but she does not seem to have the ability to keep the sacred diamonds that maintain the operation of the kingdom to continue to generate energy. For this reason, she set out under the guidance of the elders to find the legendary elf Xerneas (voiced by Yoshiko Mita). Because of her ability to make diamonds, Diancie was troubled by the female thief Marilyn (voiced by Rika Adachi), the ninja thief Leote (voiced by Hiroka Yamadera) and the Rocket Team trio on the road. At the critical moment, thanks to Xiaozhi (voiced by Rika Matsumoto), she was saved from danger. After that, the princess and Xiaozhi walked all the way, and finally came to the forest where Xerneas was. However, alongside this legendary elf with the ability to give life, there is also a terrible elf that can bring death. A brand new adventure begins...