Ash prepares for the upcoming Alola League Conference with his Pokémon. However, the collision of Torracat's Fire Blast and Lycanroc's Stone Edge triggers an explosion that whips up a powerful gust, forcing a nearby Celebi to travel through time to escape. However, it accidentally takes Ash and Torracat to the past with it. Meanwhile, Pikachu and the others find themselves suddenly abandoned and decide to search for Ash. Lycanroc spots Ash's hat moving amongst some tall grass, but the wearer turns out to be a Sudowoodo. Pikachu retrieves Ash's cap, and the group continues their search, but Rotom now finds itself alone.
Lycanroc and the others run to the Pokémon School but find no one there. They come across Meltan, who playfully runs around while wearing Ash's hat and eventually hits a wall, causing the blackboard rubber to fall onto it. This gives Pikachu an idea, and he orders everyone to go to the Ultra Guardians' base. Lycanroc slides the blackboard up to reveal the hidden control panel behind, and Rowlet activates the lift with a swift kick to the on button. The quartet enters the elevator, where they receive their Ultra Guardian symbols. The elevator ride stops, and the party is greeted by Clefable and a surprisingly large number of Meltan. Ash's Meltan invites all of the other Meltan to join it inside Ash's hat, but they threaten to tear it apart. Clefable intervenes, calling all of the Meltan to assemble before rewarding their obedience with a large bowl of hex nuts. Pikachu asks Clefable if it has seen Ash, but it hasn't. Pikachu then requests the assistance of the Ride Pokémon, and they are called over. Pikachu and Lycanroc climb onto Garchomp's back, and they lift off, while Rowlet follows them with Meltan in its talons.
At Rowlet's request, Garchomp takes them to Mount Lanakila, but it is particularly cold. Garchomp, worried about its skin, leaves Pikachu and the others to continue their search. The Pokémon ride a sled down, but the ride becomes increasingly fast and dangerous, and they try to avoid oncoming obstacles. A Glalie suddenly bursts out from the snow, causing Pikachu and the others to make a sharp turn down a steep embankment. The hill abruptly ends, sending everyone hurtling into the air. They soon land, only to continue rolling down another hill until they become a single snowball. The route leads them off a cliff, and they land into the ocean below.
Pikachu is the first to reach the surface, and soon enough Lycanroc swims over to him, followed by Rowlet, who alerts the other two that Meltan is missing. Pikachu and Lycanroc dive down to find Meltan, but all they can see is the local Water Pokémon. Rowlet flies to search from the air while the other two decide to check the ocean again. Pikachu and Lycanroc then swim directly into what they think is a cavern, but it turns out to be the inside of a Wailord. The Wailord soon breaches the surface for some air. It spouts out water from its blowholes, also propelling Meltan into Rowlet's waiting claws. Wailord then begins to struggle because of a pair of blockages. It eventually manages to build up enough pressure to clear the obstructions, causing Pikachu and Lycanroc to be spouted high into the air.
Having been blasted off by Wailord's water spouts, everyone finds themselves lying on a Melemele Island beach. Pikachu opens his eyes to find no one in sight, but he spots Ash's cap close by. As he reaches for it, the hat continues to move further away because it is on the back of a Crabrawler. After several failed attempts, Pikachu demands the hat back. Crabrawler turns around and prepares to throw a fist to keep its newfound shelter. Pikachu replies with an Iron Tail, blocking most of Crabrawler's Rock Smashes. The pair run across the water's surface as their rivalry intensifies, before returning to the sandy shores. Crabrawler and Pikachu stare at each other, with Ash's hat lying between them. Meltan stumbles upon the scene and crawls inside of Ash's hat before beginning to walk off with it. Crabrawler gives chase and snatches the hat from Meltan. However, Meltan is infuriated and retaliates with a powerful Flash Cannon. The attack causes Crabrawler to collapse and Ash's hat lands at Pikachu's feet. Pikachu congratulates Meltan just as Rowlet and Lycanroc rejoin them.
Pikachu suddenly collapses from the stress of finding Ash and the subsequent growing hunger, as do the others. However, Meltan is unfazed after finding a strewn can to snack on. Lycanroc suddenly catches a whiff of something and it leads its fellow Pokémon up a steep cliff to a Sitrus Berry-filled tree. They enthusiastically run up the tree trunk, but fail to reach any of the Berries. Pikachu lands on the ground first, only to be squashed under everyone else's weight. As a result, Pikachu zaps everyone and Meltan uses Flash Cannon, leaving them all covered in soot. For all their efforts, a single singed Sitrus Berry falls from the tree.
Suddenly a bright green light catches their attention, and they run towards it. Ash and Torracat have finally returned to their own time. He turns back to Elder, but notices that only a tree trunk with overgrown moss remains. Ash's other Pokémon rush over to greet their Trainer, greatly relieved to see him back. Ash takes his hat from Pikachu's head before placing it on his own. Ash also notices that Lycanroc and the others appear particularly dirty, but he is unable to understand their Pokémon speech replies. Afterwards, Ash leads his troupe of Pokémon along the beach and towards home.