In episode 43 (982 overall), “Gym Battle! Z-Move VS Mega Evolution!!,” Ash and his Pokemon School classmates are learning about gym battles from Misty, leader of the water-type Cerulean Gym.
During a fight between Brock’s Steelix and Kiawe’s Turtonator, the students are surprised to see their classmate use a Z-Move — a special attack that combines the full power of a trainer and their Pokémon — a strategy that’s countered by Brock, who in turn reveals a Key Stone.
The stone triggers a mid-battle evolution in Steelix, who transforms into Mega Steelix — a temporary change that lasts only for the duration of the battle, and made possible by a combination of the special stones and a bond between a Pokémon and its trainer.
Ash then faces Misty, the fiery redhead turning the field into a massive pool. Ash chooses Pikachu, only for the electric mouse to be confronted by Misty’s hulking Gyarados. As Pikachu readies an electro ball, Gyarados uses rain dance, igniting a rainstorm inside the gym before dealing a large blow to Pikachu with the destructive hurricane move.
Misty activates her own Key Stone, evolving Gyarados into Mega Gyarados. Pikachu, using its agility to evade the sea monster, avoids Gyarados’ crunch attack and counters with iron tail. Caught in a cyclone of water as result of Misty’s combining rain dance and hurricane, Pikachu fails to break free, even with thunderbolt — an inescapable situation, Misty says, revealing no trainer has ever managed to break the combo.
Ash calls for Pikachu to leap out of the hurricane, using its quick attack to ricochet off the previously futile electricity. Now freed, Ash and Pikachu team for their Z-Move, Gigavolt Havoc, which swiftly knocks out Gyarados.
Following Ash’s win, the participants are awarded commemorative gym badges, and Ash invites his old friends Brock and Misty to visit Alola.
Pokémon the Series: Sun & Moon airs Thursdays on Tokyo TV in Japan and Saturday mornings on Disney XD in the United States.