Ash, Brock and Misty are still continuing towards Mt. Silver. Larvitar is still riding in Ash's backpack, and awakens only to be greeted by Misty. Still fearful, it Hardens, leading Ash to criticize Misty's statements and baby Larvitar again. Misty complains that Larvitar doesn't like anyone besides Ash, and Brock claims that he can endear himself to the Pokémon, as he will be the world's best Pokémon Breeder. He pulls faces and tries to make Larvitar laugh, but only succeeds in terrifying it to the point of it bursting into tears. Ash coaxes it into stopping and going back to sleep, and Misty criticizes Brock's skills. With its screeching stopped, Brock hears a waterfall and they decide to find it and have lunch. Watching nearby, James and Meowth plan to steal Pikachu, only to discover that Jessie has eaten all of their lunch.
Ash and Pikachu run ahead, but end up sliding down a steep hill. His rough ride ends when he hits a boulder. Misty and Brock soon catch up and notice that they have arrived at the lake's supposed location. However, it appears to be a dried-up stream, surrounded by a number of thirsty Slowpoke. Ash and friends spot a King's Rock, but are stopped from approaching it by Alice, her father Professor Telesu, and her Slowpoke, nicknamed Arthur. Brock promptly falls in love with Alice, but Misty drags him away. Alice explains that she and her dad are researching legends surrounding a supposed Slowking who could help save the colony of Slowpoke. She explains that the lake dried up because an earthquake opened a fissure that drained most of the water, and insufficient water flows in from the river to fill it again. Professor Telesu leads them to a cliff with runes carved into it. The cliff face and the King's Rock only appeared once the water receded. Telesu explains that a special Slowpoke is destined to save the colony, by reaching the King's Rock to evolve into a Slowking. Misty notes that the legend is very similar to the events happening now, and Telesu explains that only one panel eludes him. Ash notes that it looks like a rainbow with a comb on it.
Team Rocket promptly arrives to steal the King's Rock, and Arbok and Weezing are sent out to retrieve the King's Rock. At Alice's plea, Ash sends Pikachu to intercept them. Arbok uses Poison Sting, but Pikachu evades. As Pikachu uses Quick Attack, Arbok easily dodges and knocks Pikachu aside. Just as Arbok is about to finish Pikachu off with Bite, Larvitar bursts into tears at the sight, crippling Arbok, Pikachu, Weezing and everyone around them, with the exception of Arthur, who simply wanders over and pats Larvitar on the back, stopping its tears. Team Rocket then makes a break for the King's Rock. However, Brock sends out Forretress, who trips the trio up with Rapid Spin. Pikachu then hits them with Thunderbolt and sends them blasting off.
Telesu thanks the group for driving away Team Rocket, but observes that the lake will be dry by summer. Alice says that the Slowpoke are too slow and too many to migrate to another lake successfully, and Telesu replies that the only hope is for the Slowking legend to come true. When Misty asks if any of the Slowpoke around the lake have attempted to become a Slowking, and Alice points out that one is attempting to. The group watches as the adventurous Slowpoke ambles across, but a wild Shellder leaps out of the water and it evolves into a Slowbro instead. Telesu explains that this has become a very common event. Misty and Brock note that the Slowpoke are too slow to reach the King's Rock in time. Telesu also points out that once they become a Slowbro, they cannot become a Slowking. Alice becomes very upset that the Slowpoke colony is in danger, and as Arthur watches his oldest friend cry, he decides to attempt it himself. Alice and the others call him back, but he ignores them, surprising Telesu, who says that Arthur has never disobeyed Alice. Brock realizes what Arthur is attempting to do, but Ash exclaims that Slowpoke will never make it at the speed it's taking.
The Shellder muster and begin to launch themselves at Arthur, but Arthur shows surprising agility for a Slowpoke and manages to deflect them. He slips and falls into the water, but uses his tail as a propeller to motor himself through the water and away from the Shellder. As Arthur climbs the steep rock with the King's Rock situated on top of it, a trio of Shellder leaps out of the water. As the group exclaims in horror, Larvitar bursts into tears, crippling the group and nearby Slowpoke and Slowbro, but most importantly, it forces the Shellder to retreat whilst Arthur is unaffected. He reaches the King's Rock and places it on his head, and just as Larvitar stops screeching, a Shellder bursts out and clamps onto the King's Rock, causing Arthur to evolve, becoming a Slowking. Ash takes the opportunity to check his Pokédex. Alice asks Arthur if he did this for the good of the Slowpoke, and Arthur nods.
Team Rocket realizes that they've missed out on the King's Rock, but they plan to steal Slowking instead. They fire a net, which Arthur reflects with Confusion, trapping the trio inside their balloon basket. Meowth deploys a smokescreen, but foolishly blinds Team Rocket as well as hiding them. Arthur then redirects the balloon with another Confusion, sending them crashing into a cliff face. The Shellder leap out from the water and bite onto the Rocket trio’s behinds, and Ash and Pikachu arrive to send the trio blasting off.
Back by the lake, Brock notes that Arthur is a natural leader of the colony, and Arthur turns to the waterfall, splitting the water with his powerful psychic powers. Professor Telesu realizes that the obscure panel was the split waterfall and the caves behind it. Arthur then leads the colony into the cave, with Alice and the others following behind. They arrive at a large lake and underground forest, which the Slowpoke and Slowbro move into happily.
Outside the falls, Alice tells Arthur that the Slowpoke now depend on him as their leader, and decides to release him. She hugs Arthur goodbye, and he retreats through the waterfall. As he waves farewell, the split waterfall merges back to a normal waterfall. Telesu comforts his daughter by pointing out that her own Slowpoke turned to be the chosen one, and Ash's friends note that she is lucky to have a king as her loving friend. Alice and her father bid farewell and fond thanks to the group, and Brock is dragged away by Misty as he promises to find his own paradise with Alice one day. Alice and Professor Telesu continue to overlook the paradise, and Ash and his friends continue on their journey to Mt. Silver.