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!