While making their way toward Ecruteak City, Ash and his friends find their path blocked by a river. Brock's map suggests that a bridge across exists, but all they can see is construction workers working on a new bridge. Ash asks the construction manager where the bridge over the river is; he informs Ash and friends that a typhoon had swept the old bridge away. The group is dismayed to learn that the new bridge will be completed in six months.
Meanwhile, Team Rocket soars through the sky in their hot air balloon, cooking potatoes with the balloon's fire. After Jessie complains that the meal is taking too long to cook, Meowth suggests that they turn up the heat. However, their balloon burns to a crisp, sending the occupants hurtling towards the ground. A stunned James informs the others that their potatoes are well done.
Back at the river, Ash and friends, trying to think of a way to cross the river, spy a fisherman in a rowboat. The fisherman agrees to take them across in exchange for a captured Sudowoodo, a species that supposedly lives on a nearby mountain. The friends begin looking for Sudowoodo, and even Psyduck tries to help. Surprisingly, Psyduck happens upon a Sudowoodo almost immediately. The Imitation Pokémon poses as a tree, confusing Psyduck by moving back and forth. Psyduck tries to alert Misty, but Misty doesn't see through Sudowoodo's camouflage until it moves. Discovered, Sudowoodo panics and runs, followed closely by Ash, Misty, and Brock.
After losing track of Sudowoodo, the gang enters a clearing, surprised to see a man and a woman in laboratory coats standing on tree stumps and arguing. The man introduces himself as Pierre and the woman introduces herself as Marie. They explain that they are up in the mountains to work on a research paper about the local habitat and the Sudowoodo. The two disagree about Sudowoodo: Pierre agrees with the general scientific consensus that Sudowoodo is a Rock-type, while Marie doubts that assertion, citing its Grass-type appearance and reports of the Pokémon using Fire-type attacks. Pierre seems to rely primarily on printed material, in contrast to Marie, who prefers firsthand research. Ash then gets an idea: he can use Noctowl to search for the elusive Pokémon.
While Noctowl searches, Marie informs Ash, Misty, and Brock that Sudowoodo can appear similar to many types of trees, even to the extent of changing colors with the seasons. However, Pierre immediately casts doubt on the likelihood of Sudowoodo being a Grass-type, saying they gather branches to look like trees. Then, Noctowl appears, having seen a Sudowoodo. As Ash and the rest approach the Pokémon, Sudowoodo notices them and flees, using what appears to be Vine Whip to evade Chikorita. However, Chikorita easily deflects the vine with the leaf on her head, sending Sudowoodo's "Vine Whip" into the hands of Marie, who realizes with disappointment that Sudowoodo had simply been holding a vine. Sudowoodo then uses Razor Leaf, but the leaves are all dried up, adding to the group's confusion.
Ash has Chikorita Vine Whip Sudowoodo, whereupon it suddenly lashes out with a very real-looking Vine Whip, sending Chikorita flying. Marie suggests that Ash use a different Pokémon to battle Sudowoodo in order to test its attacks again, so Ash recalls Chikorita and sends out Totodile. Both Sudowoodo and Totodile do a bit of dancing before Ash tells Totodile to Tackle the Pokémon, but Sudowoodo narrowly steps out of the way. It also avoids Totodile's Water Gun by jumping frantically about. Misty observes that Sudowoodo seems to hate the water, suggesting once again that the Pokémon is a Rock-type. Finally, Ash has Totodile attempt to Bite Sudowoodo. Sudowoodo is chased until it suddenly disappears into the ground. Ash wonders if Sudowoodo has just demonstrated Dig, meaning it might be a Ground-type, but then Sudowoodo comes back up in a net. Team Rocket emerges, disguised in tree costumes, and recites their motto. Before Ash and the others can act, James orders Weezing to use Smokescreen. When the smoke clears, Team Rocket is gone, along with Sudowoodo.
As Ash and the others search for Team Rocket and their captive, Team Rocket itself is having an argument about Sudowoodo's type. Meowth asks Sudowoodo what type it is, but Sudowoodo avoids giving a straight answer, instead saying that it wants to eat. Jessie gets impatient with the Pokémon, which acts in a similar annoying manner to Wobbuffet. Finally, they give up and decide to take the Sudowoodo to the fisherman for a ride across the river. The fisherman, suspicious that Team Rocket might be tricking him, points out that the Sudowoodo doesn't look healthy. James, assuming Sudowoodo is a Grass-type, takes out a SquirtBottle in order to heal the Pokémon. Instead, Sudowoodo cries out in fear and attempts to run away. While Jessie and Meowth hold on tight to the rope around Sudowoodo's middle, James pours some water, but the rope breaks at the last second and Sudowoodo is freed. Team Rocket chases after it, followed by the old fisherman, who is interested in catching it before they do.
Ash and the others are making no progress finding Sudowoodo. Then, Pikachu spots it standing on a rock holding tree branches, hiding from Team Rocket. Before they can escort Sudowoodo to Marie and Pierre's lab, Sudowoodo runs off once again, determined to be free. Brock gets ahead of Sudowoodo, tackles it, and tries to calm it down, assuring it that he is its friend. Then, Brock tries feeding Sudowoodo his special Pokémon food. This works, as Sudowoodo is soon begging for more food, even willing to go to the lab to receive some.
At the lab, Marie asks Sudowoodo to perform Vine Whip, but Sudowoodo nervously shakes its head, and even more of Brock's food won't convince it to try the attack. Marie suggests that Ash use one of his Pokémon to get Sudowoodo to use Vine Whip. Pikachu uses Thunder Shock, but Sudowoodo simply absorbs the electricity and beams it back, narrowly missing Pikachu. While Sudowoodo performs a victory dance, Pierre announces that his books list one of Sudowoodo's attacks as Mimic, an attack that imitates other attacks. Marie realizes that this is how Sudowoodo had used unusual attacks: it mimicked Chikorita's Vine Whip and Pikachu's Thunder Shock. To prove that Sudowoodo is indeed a Rock-type, Pierre tries to spray it with water, but Sudowoodo escapes out the window. Soon, it finds itself surrounded by Ash, his friends, Team Rocket, and the old fisherman from the river. Marie, surprised to see the fisherman, reveals that he is her grandfather. Then, the grandfather explains that he had only been looking for a Sudowoodo because Marie wanted to find one.
Determined to get Sudowoodo, Jessie sends out Arbok, while Ash then sends out Cyndaquil. Before Arbok can Tackle Cyndaquil, Sudowoodo grabs it in midair and slams it into the ground. When Arbok recovers, Sudowoodo points to the right. Arbok, falling for the trick, doesn't see Sudowoodo's Faint Attack until it is too late. It then Low Kicks the dazed Cobra Pokémon. Jessie recalls Arbok, but remarks that Sudowoodo's sly style would fit perfectly with Team Rocket. She has Wobbuffet face Sudowoodo, but Ash tells Cyndaquil to use Flamethrower on Wobbuffet before he can make a move. Sudowoodo, however, stands in front of the Flamethrower and, unharmed, sends it right back at Cyndaquil. Cyndaquil then tries to Tackle Wobbuffet, but Wobbuffet uses Counter, sending Cyndaquil flying. Misty, forming an idea in her head, sends out Poliwhirl, then asks Ash to have Cyndaquil distract Wobbuffet. Wobbuffet prepares to Counter Cyndaquil's Flamethrower, but then, Poliwhirl hits him with a Water Gun, blasting Team Rocket off.
Meanwhile, Sudowoodo runs around, trying to avoid the drops of water from Poliwhirl's attacks. It flees once again, but Marie tells the others to let it go, since Sudowoodo is most certainly a Rock-type from its behavior. Pierre comments that he is glad Marie questioned the established position, since if she hadn't, they never would have learned so much about it. Pierre and Marie apologize to each other for their arguing; and then thank Ash, Misty, and Brock for their help.
Later, Ash and his friends cross the river to Ecruteak City on Marie's grandfather's boat. Meanwhile, as Team Rocket is desperately climbing a cliff, James grabs on to what he thinks is a tree branch. However, as he pulls himself up onto the edge of the cliff, James sees that it is a Sudowoodo's arm he has been holding onto. Sudowoodo shakes Team Rocket off, leaving them to plunge down the cliff.