Set in the mid-1970s, the film is based on the true story of Yoshiharu Yamaguchi, a teacher and former Japanese international rugby union player, who turned-around a failing high school in Kyoto, giving special attention to the rugby team, which included the most disruptive students among its members.

In the underground Diamond Domain, where many Carbink live, the Mythical Pokémon 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 Pokémon Xerneas, Diancie encounters a group of thieves who want to take control of its diamond-producing power–and who awaken the Legendary Pokémon 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?