The Sun, the Scare, the Secret Lair! - S20.E12

Professor Kukui takes Ash and the other students for a special lesson at the beach, where they come across a bunch of Mareanie. One Mareanie causes trouble for Ash and his Pokémon when it decides to help Team Rocket (and especially James), who are under pressure from their boss to catch new Pokémon in the Alola region.