Hapu sent Mudsdale to confront some Team Skull grunts who were stealing several of her Poni radishes. The latter had Zubat, Garbodor and Salandit use Supersonic, Venoshock and Flame Burst, but Mudsdale, due to its Stamina ability, raised its defense from these attacks. To retaliate, Mudsdale used High Horsepower and defeated the grunts' Pokémon in one attack. Mudsdale was with Hapu at the Ruins of Hope, where she wanted to contact the spirit of her grandfather. As Hapu failed to do that, she went out of the ruins, but Mudsdale stepped in something and became upset. Hapu tried to calm it down, but Ash, who came near the ruins, had Pikachu use Iron Tail at Mudsdale's hoof. The knocked off item was a Corsola's horn, and Mudsdale calmed down. Ash accompanied Hapu, until the latter became offended when the latter had no idea what Grand Trials represented, and had Mudsdale kick him away. Mudsdale later watched Ash and Gladion's battle between Dusk Lycanroc and Midnight Lycanroc.
When Sophocles, Lillie and Mallow, who visited Hapu, left, the latter sent Mudsdale and Golurk out, pretending that she didn't need company. When Tapu Fini engulfed Ash, Lillie, Gladion and Mallow, Hapu went on Mudsdale to the Ruins of Hope, to plead to the Guardian Deity to release her friends. She later returned to her friends, who were freed.
Before the ceremony to become an Island Kahuna, Hapu loaded some Poni radishes on her Mudsdale. On the night of the Grand Trial, Hapu, as promised, sent Musdale, while Ash sent Pikachu. Pikachu used Quick Attack against Mudsdale, which had little effect, due to Mudsdale's Stamina ability. As Mudsdale continued taking the attacks, it eventually retaliated with High Horsepower and collided with Quick Attack. Pikachu went to use Iron Tail, but was stopped and hit by Musdale's Double Kick. To continue, Mudsdale continuously used Bulldoze, causing the ruins' boulders to fall into the sea and soak Ash's friends. Thus, to change the tide of battle, Hapu used the power of her Groundium-Z, allowing Mudsdale to use Tectonic Rage Z-Move. Pikachu ran off from Mudsdale, but got rammed high-up in the air. Since he could take a lot of falling damage, Pikachu used Electroweb on the ruins to bounce himself off into the sea, shocking Hapu for that move. Pikachu, who returned to the battlefield, used Quick Attack to circle around Mudsdale, who attempted to inflict damage through Stomp. Once at the edge, Mudsdale used Bulldoze, but Pikachu jumped away from the pillar, which fell into the sea and soaked Mudsdale. Since Mudsdale was wet, it became an easy target for Pikachu's Z-Move, Gigavolt Havoc, which electrocuted it and defeated it.
When a Guzzlord descended from an Ultra Wormhole at Manalo Stadium, Hapu sent her Mudsdale, along with the rest of the Island Kahuna to battle it. Mudsdale used Bulldoze to push Guzzlord away. However, Guzzlord retaliated by uttering a loud howl to blast the Island Kahuna away. To retaliate, Hapu, along with the rest of the Island Kahuna, used their Z-Rings to power their Pokémon up. Mudsdale used Tectonic Rage, and with the rest of the Z-Moves, it pushed Guzzlord through the Ultra Wormhole, which sealed itself.