While their friends visit Kiawe’s farm, Lana and Ash decide to go fishing. At Olivia’s suggestion, they head to Brooklet Hill, where a Totem Pokémon is rumored to live!
After helping an injured Wishiwashi and catching a Feebas to prove herself to the master fisherman who guards the lake, Lana manages to hook the enormous Totem Pokémon, and the battle begins! The Pokémon is strong, but Lana hangs on with fierce determination—and in its own determination to help her, Popplio learns Aqua Jet! Defeated, the Totem Pokémon scatters into a school of Wishiwashi—and one of them presents Lana with a Water-type Z-Crystal to use with her new Z-Ring!
Team Rocket are fishing at Brooklet Hill, trying to catch a Totem Pokémon based on information that James had received from a man who calls himself the "master fisherman". Meowth reels in a small Pokémon, which James identifies with his book as a Wishiwashi. Meowth, unimpressed, promptly throws the Small Fry Pokémon away. Jessie's then feels a pull on her fishing rod, and she attempts to reel in what she thinks is the Totem Pokémon. However, Bewear shows up out of the water, and Team Rocket blasts off into the air.
In Paniola Town, it is extracurricular activity time for the heroes, and the group are free to go wherever they want. Ash joins Lana, who reveals that she will be going fishing to catch the biggest Pokémon. Lana then lends Ash a fishing rod so that he can join in the friendly competition. The rest of the group then join the two, and reveal that they are going to Kiawe's farm, on the premise that they will be helping him with his work.
As Ash and Lana exit the Pokémon Center, they meet Olivia, who greets them just as she falls from a Tauros. After getting back up, Olivia reveals that by the end of the day, she would have finished Lana's Z-Ring. Also, as the pair are going fishing, she mentions that there is a large Totem Pokémon residing in a lake near Brooklet Hill. Excited, the pair head off to catch the Totem Pokémon.
At Brooklet Hill, Ash and Lana are ready to begin fishing, but get interrupted by the "master fisherman" who states that they will need his permission before battling the lake's Totem Pokémon. He then recounts an exaggerated tale of battling the Totem Pokémon, showing a fake scar on his arm as “proof”. He agrees to let them fish if they manage to find a Feebas, which Lana, to his astonishment, reels in almost immediately. They then exchange Feebas to rent a small boat which the man says will be necessary in catching the Totem Pokémon.
Before they head off on the boat, Lana discovers an injured Wishiwashi stuck in a small, shallow patch of water. Lana bandages its injuries and then lets it go into the lake. The two then head off and begin fishing in the middle of the lake. Ash reels in a Magikarp, which proceeds to slap Ash on the face multiple times before fleeing. Lana then senses a big catch and manages to get a bite. The bite reveals itself to be the Totem Pokémon, a School Form Wishiwashi, however Rotom announces that it is an unknown Pokémon.
Meanwhile at Kiawe's farm, rather than assisting Kiawe, Mallow and Sophocles are scoffing down some cheese and ice cream respectively (made specially from the farm's Moomoo Milk), while Lillie is feeding Snowy some special-made Pokémon food.
Back at Brooklet Hill, Ash and Lana's boat is being pulled across the lake by the large Wishiwashi at the end of Lana's fishing rod. The boat then collides with an island in the middle of the lake, and while Lana manages to land properly, Ash and Pikachu are sent flying into the lake. Seeing Wishiwashi approaching them, Ash manages to quickly jump onto the island but Pikachu remains floundering in the water. At Lana's command, Popplio manages to save Pikachu from the Wishiwashi in time.
The struggle between Lana and Wishiwashi continues, and Popplio dives in the water and attacks with Bubble Beam. But as the Totem Pokémon approaches, scared, Popplio jumps out of the water and Lana decides to handle the battle by herself. The struggle between Lana and the Totem Pokémon continues, but the Totem Pokémon is too strong, and sends Lana struggling on her knees and against a rock face. Seeing its Trainer losing, Popplio overcomes its fear of the Wishiwashi and dives in to battle. After firing Bubble Beam several times, Lana begins to regain her control of Wishiwashi. However, Popplio is attacked by a nearby Pokémon, revealed to be an ally Alomomola. Alolomola then commences to use Heal Pulse on the Wishiwashi. Ash decides to handle the situation by distracting Alomomola with his fishing rod, and manages to reel it in. Pikachu then attacks Alomomola with Electro Ball, causing the Caring Pokémon to flee.
Popplio, now back in action, continues its battle with the Totem Pokémon. After racing in front of the Totem Pokémon across the lake, Popplio manages to defeat the Wishiwashi with its newly learnt Aqua Jet, managing to break up the school of Wishiwashi. The Solo Form Wishiwashi flee, and the Wishiwashi that Lana had helped before then appears out of the water. In its mouth is a Waterium Z, which it hands over to Lana.
At night in the Pokémon Center, Olivia hands Lana the finished Z-Ring, and Lana fits her Waterium Z into it.