Roll On, Pokémon! (Japanese: ドンファンのたに！The Valley of Donfan!) is the 120th episode of the Pokémon anime. It was first broadcast in Japan on November 4, 1999 and in the United States on November 4, 2000.
Ash and friends stumble into a small valley where Donphan are used to harvest precious stones. Not so fortunate are the valley's residents, as Team Rocket is on the heels of Ash and his friends and are already hatching a plan to steal the stones for themselves.
Ash and his friends come across a scenic valley, which they must pass in order to reach Violet City. As they continue onward, the ground starts to shake and some large rocks fall from the cliff above. Meanwhile, Team Rocket is sitting by the ledge on the same cliff. When more tremors hit, the rock they are sitting on crumbles beneath them, and the trio falls into the forest below, with tree vines helping to break their fall. Ash, Misty, and Brock continue along a dirt trail, when they stumble upon a Donphan amongst the shrubs. Ash is about to catch it when Brock stops him, saying that it must be young due to the short size of its tusks. Brock then suggests catching Donphan because it needs a Pokémon Breeder. Ash and Brock then start arguing over who would be the best Trainer for Donphan, only for Misty to cut in and tell them that Donphan is walking away.
At this, Brock calls out Onix and commands it to use Tackle on Donphan, sending it backwards. However, Donphan soon recovers by curling up into a ball and rolling at Onix, hitting and defeating it. Ash then sends out his new Heracross, which uses Tackle, sending Donphan flying backwards. Donphan jumps back up and charges forward with another attack. Heracross uses Horn Attack, which hurtles Donphan into a tree trunk. As Ash is about to order his finishing attack, a girl intervenes. She checks on Donphan's health, then threatens to turn Ash in to Officer Jenny for attempting to steal her Pokémon. Ash apologizes for the misunderstanding and explains that he and Brock thought that Donphan was a wild Pokémon. At that moment, Brock starts flirting with the girl, who identifies herself as Rochelle. Disapproving of Brock's advances, she pulls away as Brock comments on her amberite necklace. Rochelle informs the group that all of the Donphan are hers, and Ash agrees to desist from trying to catch any of them. After denying Ash's request for a battle, Rochelle walks into the trees and disappears.
Elsewhere, Team Rocket has donned orange ranger outfits to avoid suspicion as they plot to mine the valley for rare amberite. Meowth explains that he overheard a conversation back at a mountain rest stop, which revealed that Donphan are able to "sniff out" amberite. At that, Jessie and James set their sights on finding a Donphan to aid their efforts. The trio steals Rochelle's Donphan by capturing it in a net and flying away with it in their Meowth balloon. However, Donphan does not immediately obey Team Rocket's orders. James tries a different approach and offers it a meal of plants and leaves that he collected. Donphan accepts the bribe and begins to help them. Their efforts to find the amberite are not fruitful, however, as Donphan is unable to find any amberite in the area.
Ash and his friends stumble across Rochelle again as she tries to locate her missing Donphan. With Rochelle's description, Ash and the group realize the culprits are Team Rocket, and they agree to assist Rochelle. After Zubat cannot find the stolen Donphan with Supersonic due to interference from the trees, Rochelle blows a high-pitched whistle. Almost immediately, a herd of Donphan appears by her side. It is revealed that Rochelle has not trained the stolen Donphan to respond to the whistle, since it is too young and she has not had enough time to do so. The Donphan are ordered by Rochelle to find the missing herd member. Shortly after, the group locates Team Rocket, who recites their motto and sends out Arbok and Victreebel. Brock calls on Onix for assistance and holds Victreebel in a bind, but it is put to sleep by Victreebel's Sleep Powder. Ash and Misty promptly send out Heracross and Poliwag, respectively. Poliwag Double Slaps Victreebel and follows up with Bubble, while Heracross Tackles Arbok, sending it flying into Jessie. With a Horn Attack, Heracross tosses Team Rocket into their Meowth balloon. To finish things off, Pikachu strikes them with Thunderbolt, causing the hot-air balloon to burst and sending Team Rocket flying off into the distance.
Relieved to have her Donphan back, Rochelle agrees to show Ash and his friends a demonstration of her Donphan's amberite-finding skills. After taking a quick sniff of Rochelle's necklace, her most experienced Donphan charges at a nearby cliff. Ash and his friends soon realize that it was the Donphan causing the tremors. Suddenly, some rubble falls to the ground and smashes open to reveal the amberite inside. Rochelle picks up the sample, and explains that she supplies local artists and jewelers with amberite. As thanks, Rochelle agrees to a one-on-one battle against Ash. He selects Heracross, while Rochelle agrees to use her most experienced Donphan. Donphan starts off with Tackle, but Heracross uses Endure to resist the attack and tosses its opponent out of the way. Donphan recovers and strikes Heracross with a Rollout from behind. Heracross is sent into a tree and is momentarily distracted by its sap. Donphan tries another Tackle, but Heracross negates the attack with its large horn. Ash sees his opportunity and has Heracross toss Donphan into the air. However, Donphan uses Rollout and then knocks Heracross out with Tackle. Ash runs over to Heracross' side, but is unfazed by the loss. Rochelle commends Ash on his teamwork and Heracross' strength, while Ash is grateful for the battle experience. At sunset, Ash and his friends say their goodbyes to Rochelle and her Donphan, as they continue on their journey to Violet City.
Brock's Zubat was depicted with the backs of its wings being the same in appearance as the insides of its wings. It may be worth noting that although the back of Zubat's wings are different in appearance to its wing insides, Golbat's wings are not so.
Brock calls Ash selfish, and that he should think about what is best for Donphan, which Ash retorts that he's what's best for Donphan. Originally, Brock mentions that Ash already got Heracross and Brock should have the honor of capturing Donphan, which Ash responds with his wanting to capture it, since he saw it first.
The substance that Donphan were tasked to look for was actually Agate, not Amberite.
Rochelle's reasons for why Ash and his friends shouldn't try to capture the Donphan was slightly different. The dub implies that the entire lot of Donphan belonged to her and her alone, while the original makes it clear that they belong to other Trainers besides herself.
Meowth's statement about where he heard about Amberite/Agate was "at a rest stop in the mountains" in the dub and an Odango shop in the original.
Jessie refers to the Japanese idiom Hidari Uchiwa, with James commenting about Jessie's fan being on her right hand, not her left. The dub replaces it with James asking about the fan and Jessie retorting about the hot air.
Brock mentioned in the dub that Rochelle must have been pretty impressed by them, with her congratulating Pikachu more specifically. In the original version, Brock was actually expressing his happiness that the Donphan were safe, and that Rochelle was thanking Ash and his friends in general.
Ash originally asks if Heracross was all right, and Rochelle states that she thinks she won.