Our heroes are camped out by a lake where Ash wakes up one morning and sees a Rhydon emerging from the water. A swimming Rock-type Pokémon seems impossible, but one of the local Rhydon has indeed learned to use Surf—and a girl named Pietra is out to catch it! She's trying to complete a tunnel to a nearby town, but Rock-type Pokémon flee the excavation site whenever water drips from the tunnel's ceiling. If she had that swimming Rhydon, though, it would keep digging the tunnel no matter what!
Ash and his friends offer to help, of course. After chasing a wrong lead or two, they catch up with Rhydon in the middle of the lake. Pietra asks Rhydon to battle her, but Rhydon couldn't care less; it learned to swim so it could reach an island full of apple trees, and right now those seem much more interesting. Just as they catch up with Rhydon again, Team Rocket descends from the skies, grabs Rhydon, and flies away. They don't get far before Ash and his friends stop their escape, free Rhydon, and send them packing.
Rhydon just wants to leave, but Pietra clings to it and demands a battle. She refuses to let go even when Rhydon dives below the water, so it takes her to shore before walking away. Pietra pleads for Rhydon's help one last time and Rhydon finally agrees to battle, though it takes some hard work on Marill's part to weaken it so Pietra can make a capture. With Rhydon's help, the excavation is soon complete. Ash and his friends exit the newly completed tunnel and continue on towards Olivine City!
Pikachu and Togepi wake up just as the sun rises. The pair venture out of the group's tent and spot a strange horn in the nearby lake. Pikachu calls out to Ash, and he arrives to see a Rhydon emerge from the water. Ash tells his friends, but Misty affirms that a Rock-type Pokémon that loves swimming is unheard. A young girl, Pietra, overhears the conversation and confirms that there is, in fact, a swimming Rhydon. Brock is quick to flirt with her, but Misty pulls him aside by the ear. Pietra informs everyone that while a swimming Rhydon is unusual, this particular Rhydon has learned to use Surf. She admits that she is keen to catch the Rhydon and shows the group to a nearby tunnel. Pietra explains that she is hoping to catch it to complete a tunnel to allow her grandma to access the nearby town. Ash notices Pietra's Dugtrio moving lumps of soil out of the way. Pietra points out that all of her Ground-type Pokémon flee from the tunnel whenever water hits them from leaks in the ceiling. Pietra thinks that if she could catch the swimming Rhydon then it could help her complete it. Brock is keen to impress and calls upon his Onix for assistance. The Rock Snake Pokémon smashes its head against the hardened rock several times, but it quickly flees after some water hits its back. After Brock's failed attempt, Ash offers to help Pietra catch the Rhydon. Team Rocket eavesdrops on the conversation and are keen to snatch the swimming Rhydon for themselves.
Pietra takes the group to the site of several Rhydon footprints. Misty asks why she hasn't just followed the tracks, but Pietra explains that the task is more difficult than that. Everyone comes to understand, as the tracks abruptly end at the lakeside. Ash spots an apple core on the ground and Pietra explains that the Rhydon loves to eat the fruit. They decide to look for some apple trees in the search for Rhydon. Team Rocket is close behind and also has the same idea. The trio spots a set of round footprints, but instead, they lead them to Jessie's Wobbuffet who happily snacks on an apple. Team Rocket spots the apple-filled tree and decides to take a snack break as well, but this is interrupted when Ash and his friends discover them. The trio performs their motto while gorge on the apples, only to choke near the end. The inaudible rendition leaves Ash and his friends confused. Suddenly Rhydon appears, and it appears to be angry at Team Rocket for eating its apples. It charges at them with its horn and sends them blasting off.
Pietra is keen to battle Rhydon, but it makes the ground shake with three powerful stomps before rushing off. As Rhydon dives into the lake, Ash sends out Totodile to pursue it. Pietra calls on her Marill, and Misty's Water-types also join the search party. Pietra loads everyone else onto her speed boat, and they sail into the lake. However, the search is not going too well until Totodile bumps into a strange object while performing a backstroke, and it instinctively clamps onto it. Rhydon rises from the water and spins its horn to throw Totodile off. The remaining Water Pokémon surround Rhydon, but Rhydon flees once again. The fleet of Water-types catch up to Rhydon but are then caught up in a whirlpool whipped by Rhydon spinning its horn underwater. The group's Pokémon are soon defeated and float to the surface, forcing Pietra and the others to return to the shore. As the group considers their next plan, Misty spots Rhydon walking onto a nearby lake island. Brock's binoculars reveal that the island also has a large supply of apples.
Team Rocket overhears everything and makes their way over to the island. Soon, James and Meowth put the finishing touches on their latest pitfall trap, covering it with branches. The pair notices Jessie snoozing away, and they decide to join her while they wait for their catch. A loud crash instantly wakes Jessie up, and she furiously demands her teammates stop slacking off. The trio inspects their pit, but they are disappointed to see that they only caught Wobbuffet.
Pietra and the others lay in wait for Rhydon. Their target soon walks by and Pietra demands a battle, but Rhydon dismisses her challenge. Pikachu delivers a Thunderbolt to get Rhydon's attention, and this allows Pietra to talk things over with it. However, Team Rocket net Rhydon for themselves and begin to soar into their sky in their Meowth balloon. Brock sends out his Crobat, and its Wing Attack slashes Team Rocket's balloon, causing them to plummet into the lake below. However, Team Rocket emerges unscathed and in the safety of their Wobbuffet life raft. As James begins to row away, Pietra jumps into her speedboat with Ash's group aboard. A Razor Leaf from Ash's Bulbasaur frees Rhydon from its capture net, and it promptly leaps into the lake. Pikachu sends Team Rocket on their way with another Thunderbolt attack.
Pietra is determined to have a battle with Rhydon and jumps into the water after it. She latches onto Rhydon's neck, and while Rhydon dives down into the water, Pietra continues to cling on. In a different tactic, Rhydon breaches the surface and continues to swim off, though Pietra's grip remains firm. Brock takes control of the speedboat, and they make it to the lake island as Pietra eventually loses hold of Rhydon. Pietra pleads with Rhydon and Marill also makes a firm stance, and upon seeing this Rhydon turns around to accept Pietra's challenge. Rhydon makes the first move, but Marill dodges its tail swing. Marill attempts a Water Gun, but Rhydon uses Horn Drill to defend itself. Pietra orders a Tackle attack, though Rhydon's thick hide bounces the little Pokémon into the ground. Rhydon prepares to stomp on Marill, but Pietra has Marill retaliate with an effective Bubble Beam. Marill takes a hit from Rhydon's tail. But follows up with a Water Gun that knocks Rhydon to the ground. Seeing an opportunity, Pietra throws a Poké Ball at her target. The Ball rocks for a moment until Rhydon emerges again. Rhydon is more determined and uses Horn Drill to block a Bubble Beam and Water Gun attack. Marill narrowly dodges a forward charge before clubbing Rhydon with an Iron Tail to the face. Rhydon hits the ground again. Pietra tries another Poké Ball, and this time the swimming Rhydon has been captured. Pietra is pleased and thanks Marill for its assistance.
The group and Pietra venture back to the work-in-progress tunnel. Pietra calls out her newly-caught Rhydon to break through to the other side. As expected, Rhydon is unfazed by the trickling water. With a Horn Drill and a follow-up Take Down attack, Rhydon smashes through the stone wall. Everyone gazes at the light flooding the tunnel, and soon Ash and his friends continue on their way towards Olivine City.