Ash and his friends have come to Poni Island to meet the legendary Pokémon Solgaleo. However, there's no Solgaleo there; instead, the Pokémon said to be the guardian deities of the Alola region have gathered there. And just before the surprised Ash's eyes, Tapu Koko and the other Pokémon take a certain action.
Ash and the others have passed through the Ultra Wormhole thanks to Solgaleo's powers. The place they eventually arrive is the world the Ultra Beasts live in. They make an attempt to rescue Lusamine, but the Pokémon that have been put under the Ultra Beasts' control stand in their way.