Ash and his classmates make their way towards the ferry bound for Melemele Island after finishing their stayover tour of the Hokulani Observatory. Poipole flies ahead and notices a shortcut sign and Sophocles adds that the cave route will lead directly to the ferry terminal. Convinced, the group venture into the cave system which turns out to be illuminated by glowing moss. Sophocles notes that the cave also leads to Mount Lanakila. Kiawe becomes concerned that they could get lost, but Sophocles is confident he knows his directions. Suddenly, Marowak begins chasing the other Pokémon with its luminous moss-covered bone. Turtonator attempts to intervene, but it is knocked onto its shell causing an explosion which sends the group and their Pokémon flying in multiple directions.
All of Ash's Pokémon, except Rowlet, end up together, and then, they end up getting flattened by Turtonator. Lycanroc sniffs the ground and takes the lead. Sophocles, Lillie, and Mallow have all of their Pokémon with them, while Lana and her Popplio find Rotom and Marowak nearby. Meanwhile Ash and Kiawe attempt to find everyone else. Kiawe tells Ash to watch where he steps, but the latter ends up stepping on a sleeping Boldore, angering it. Ash tries to send Rowlet to fight, but it won't wake up, forcing Ash and Kiawe to flee.
Elsewhere, Lillie puts her finger in the air in an attempt to find a wind current that could lead to an exit, and Sophocles and Mallow do the same. The three feel a breeze coming their way, but it turns out to be a rabid Alolan Raticate, that is rampaging towards them, forcing the trio to run. The trio escape but are exhausted. Fortunately Mallow has some supplies on hand, and Sophocles, Lillie, and the Pokémon enjoy the homemade cookies. Lillie then notices Mallow isn't eating, though Mallow explains that the look on a satisfied person's face is irreplaceable and that she hopes that she can make more people happy through her cooking in the future. Mallow then asks Sophocles about his dream. He admits his fascination with the stars has led him to want to become an astronaut. While Lillie explains that she hopes to become closer with Pokémon and doesn't have a particular dream in mind. Snowy's ears switch, and the group soon notice a group of Alolan Sandshrew in the midst of training. Sophocles jumps to stop his Togedemaru from disrupting the group, but his landing alerts the wild Pokémon to the trio's presence. He tries to apologise, but the Sandshrew soon surround them. Mallow notices one of the Sandshrew sniffing the air around her, so she gives it one of her remaining cookies. It quickly grabs the treat and eats it.
Lana has arrived at a fork in the cave. Rotom's 50/50 calculations prove useless so Lana just decides to go right. Meanwhile, Kiawe and Ash are exhausted as the former mentions that Ash ended up stepping on a Diglett and a Gumshoos. Ash suddenly spots an exit and the pair run out directly into the blizzard conditions. They quickly realize they have made gone the wrong direction and ended up at Mount Lanakila. Elsewhere, the Sandshrew have begun to calm down after a small snack, much to Sophocles, Lillie and Mallow's relief. An unusually large Sandshrew approaches Lillie for a pat. Its ears twitch, and the Sandshrew begin fleeing and the trio follow suit at Snowy's insistence.
As they are running, Lillie notices that the ground is becoming colder and they soon reach their destination, a large ice structure that resembles a castle. They watch on as the Sandshrew confront an intimidating Tyranitar. Lillie notices the claw marks across the cavern's ceiling and walls, and suggests the territory must belong to the Sandshrew. Snowy is blown away by Tyranitar's Sandstorm and another attack causes the cavern to become unstable. Lillie and Snowy becomes separated by the debris. She is fine, though is more concerned about the Sandshrew's conditions. Meanwhile the large Sandshrew continues to battle on and evades another Stone Edge from Tyranitar, but the attack does more damage to the cave. Lillie rushes into the conflict while Sophocles enlists his Togedemaru and Charjabug, Mallow and her Steenee to help dig a path to reach her. Lillie reaches the battle as Sandshrew is winded by another Stone Edge. Snowy launches a Hail, and as predicted the change of conditions give the Sandshrew favourable hope as Lillie attempts to speak to Tyranitar. Lillie thinks back to Ash's match against Olivia and decides a Double Battle is in order. Snowy uses Aurora Veil which protect it and Sandshrew from Tyranitar's attack. Lillie follows up with a Powder Snow and at her command Sandshrew unleashes several Rapid Spins. Mallow and Sophocles are glad she has Tyranitar cornered and cheer Lillie on. Sandshrew uses Metal Claw which topples Tyranitar to the ground. Tyranitar tries to get up, but flees the cavern after Sandshrew roars. Sophocles and Mallow are amazed by Lillie's command of the wild Pokémon and her victory. However Lillie brushes off the praise by stating that it was a spur of the moment thing, and it was the Pokémon that made the effort. The trio watch on as the Sandshrew get back to constructing their nest.
Meanwhile, Ash and Kiawe emerge from the cave system and encounter Lana there. Kiawe is overjoyed to be reunited with his Marowak, while his Pokémon is overwhelmed and begins to tear up. Rotom floats into Ash's arms, and Ash's other Pokémon soon make an appearance.
Lillie has Snowy freezes over a cavern entrance to prevent any more intruders from entering the Sandshrews' nest. Charjabug suddenly excavates an Ice Stone which Sophocles explains can be used to evolve either an Alolan Vulpix or Alolan Sandshrew. The large Sandshrew nudges the Ice Stone in Sophocles's hand and goes on to evolve into an Alolan Sandslash. Lillie is glad it evolved and can now defend the Sandshrew with its newfound strength. Sandslash walks off somewhere and returns with an Icium Z, which it presents to Lillie as gratitude for her and Snowy's assistance. Lillie is unsure because she doesn't have a Z-Ring yet, but Sophocles and Mallow encourage her to take it anyway, stating that she might get a Z-Ring in the future, and with that, Lillie accepts the generous gift.
Sophocles, Lillie and Mallow emerge from the cave where they announce to their fellow classmates that they had a fantastic adventure. The claim instantly piques the others' interests. Sophocles goes on to boast that they found a large ice castle, while Lillie reveals that she succeeded in a Double Battle and earned herself an Icium Z. As they admire Lillie's reward, Charjabug reveals that it has found another Ice Stone. Sophocles hands it to Lillie so she can evolve Snowy. Lillie formally offers the Ice Stone to Snowy, but it cowers away from it. Seeing her Pokémon's reaction, Lillie deduces that Snowy is not ready to evolve yet and respects its decision. While the others imagine Lillie's future Z-Move and Alolan Ninetales, she smiles at the possibility.