At long last, Black, who is obsessed with winning the Pokémon League, receives his Pokémon, Tepig, and begins to explore the Unova Region and fill one of Professor Juniper's Pokédex.

In the underground Diamond Domain, where many Carbink live, the mythical Pokémon Diancie serves as a princess. The Heart Diamond that sustains the land is beginning to fall apart, and Diancie is not yet strong enough to create a new one. Can Ash and his friends help Diancie discover its true power and save the Diamond Domain.