Ash, Misty, and Tracey are walking through a mountainous desert region that looks like Monument Valley. Ash is tired and hungry. He offers to eat if his companions are tired and hungry too. Tracey then calls to Ash and Misty to see what he is seeing. They run up to Tracey, and they discover that they have come upon the awesome metropolis that is the city of Trovitopolis, the largest city on Mandarin Island South. Jessie, James, and Meowth spy on Ash and his friends as this time, they have cooked up what they think is the perfect plan, in which they are actually standing a large trap, as they have put dynamite under the ground and once detonated, they will have trapped Pikachu. Meowth prepare to activate a trap button. But it doesn't work. Meowth wonders where the explosion is, and then the ground disappears underneath them and they fall into a dark hole as deep as an ocean abyss! Misty is heard screaming. Tracey wonders why anybody would be screaming. Suddenly, the ground starts to crack and it collapses right underneath them, and they too fall into a hole as deep and dark as an ocean trench! Ash and Misty are woken up by Pikachu and Togepi, but Tracey is stuck in some rubble. So Ash, Misty, and Pikachu pull him out. The group look down the cave, all stand up and start to walk down the cave. As Ash and friends are walking through the sewer, a bell-ringing sound is heard somewhere. Then, a Vine Whip comes from the water and taps on Ash's shoulder. Ash thinks that it was Pikachu and begs him to stop. Pikachu isn't on Ash's shoulder, but on the ground in front of him. Another vine appears and the group sees this. Ash commands Pikachu to use ThunderShock, but Misty states that if Pikachu uses an electric attack, everyone will be shocked, so Ash sends out Bulbasaur. He tells it to use Razor Leaf, but the creature hits the leaves away, and then grabs Bulbasaur and pulls it into the sewer water! Ash attempts to jump in after it, but Misty, Tracey and Pikachu hold him back. Tracey points out that he will be dragged down too, but Ash does not care and still attempts to jump in to save him, As they they struggle for a while, and Officer Jenny comes in with a light up near him Jenny explains that the sewer is very dangerous and throws a rope down to them. But Ash refuses, wanting to find Bulbasaur, so Misty and Tracey tie him up and Jenny then hoists him. Ash and friends are thrown in jail and Ash bangs on the cell door. Misty and Tracey wonder why they were thrown in jail for walking in the sewers. Jenny comes in to inform them they are free to go. Then a large man with a large mustache enters, and hopes that Jenny is not releasing them. Jenny then acknowledges the Mayor of Trovitopolis's presence. She tells him that they only arrived in town today and are going to the Pokémon Gym on Trovita Island, and that their story checks out. The Mayor can't have this incident leaking out yet and says she should keep them locked up until the next week. Officer Jenny of Trovitopolis tells him that she can't hold them if they aren't charged with a crime. The Mayor has to accept that because it is the law, but he orders them not to say anything about the creature. But Ash tells the Mayor that all he wants to do is get Bulbasaur back. This makes the Mayor lose his temper and threaten to have them all arrested for endangering public safety. Jenny reveals to them that they are not the first people to have seen the creature. The Mayor is reluctant, but Jenny tells them anyway. She reveals to them that the police force has had a number of reported encounters with a mysterious creature in the sewers, but no serious injuries reported. She then tells them that the Mayor says people will panic if word gets out about the creature, because they will never be able to capture it. Ash asks why, and Jenny reveals that there are almost 500 miles of sewer under the city, and over 5,000 manholes. The Mayor then says that is why they have to take drastic and immediate action to ensure the safety of everyone in the city. He also says that there is an election next week and if word gets out that he is letting a monster roam the sewers then he won't stand a chance. Nurse Joy comes in and tells him that she has heard what he intends to do about the sewer creature problem. The Mayor tells her that is his business, and group ask Nurse Joy about his plan. She tells them that instead of capturing the creature so it can be studied, the Mayor intends to send a SWAT team down to the sewers to destroy it. Ash, Misty, and Tracey respond by saying the Mayor's plan is cruel and that it could be a whole new species of Pokémon. The Mayor says that any talk of any such plan is pure speculation, and that there is no evidence. Jenny tells him that such a plan is illegal and the police force don't know anything about it. The Mayor loses his temper again and proclaims that he is the Mayor, he is responsible for the city and he will do what he sees fit for the city. At the Pokémon Center, the group speak to Professor Oak about the new species. Ash asks Oak what they should do, and Oak says that there has to be some way of preventing the Mayor from destroying the creature. Nurse Joy does not think so, because the only thing the Mayor cares about is being re-elected, and he's not going to let anything get in his way. The group see dozens of campaign posters all over building walls and park fences, and see a statue of the Mayor. The group notice "M" symbol on the foundation of the statue, and recognize it from all the campaign posters they saw earlier. Ash has had enough and decides to return to the sewers to search for Bulbasaur. The group finds that Mayor of Trovitopolis is briefing his SWAT Team inside the sewer. Ash knows how to find an entrance. Nurse Joy gave them another entrance to the sewer on the other side of the city. Meanwhile, Team Rocket ends up tired after falling into the sewers at the beginning of the episode. All Meowth wants to do is eat, but James says there is nothing to eat down where they are. They hear the same bell-ringing sound, and the creature reappears and starts rubbing Meowth's chin. Meowth mistakenly think that Jessie or James rubbed him, but he finds that it wasn't them. The creature appears and Team Rocket run away in panic. Team Rocket ends up exhausted again. They then hear footsteps, and a flashlight is pointed at them. James sends out his Weezing and commands it to use SmokeScreen, as it engulfs the sewer with smoke. Team Rocket then make their getaway, screaming in fear. It turns out that Ash was holding the flashlight and Misty and Tracey are there too. Ash asks if everyone is alright, then Misty notices that Togepi is missing. They spot Togepi at the end of the path. Misty chases after it, and she picks up Togepi. The creature appears and grabs Togepi and pulls him on the sewer water! Misty tries to water to save him, but Ash and Tracey pull her. Ash decided to use his secret weapon he asked Professor Oak to send him; Muk. Muk Body Slams Ash in affection. Meanwhile Team Rocket are exhausted for a second time. Jessie and James decided to use a scary disguise. Jessie split her hair in two, and Meowth is biting onto James's head. They then come across none other than the Mayor's SWAT team. The Squad Leader radios in to the Mayor, and tells him that they have discovered 3 unidentified creatures. The Mayor compliments them on their excellent work, and commands them to attack immediately. The SWAT open fire at Team Rocket with puddy guns, running away in the process. The scene cuts back to Ash and Tracey, with Muk in front. They come to a dead end, and Muk points out that they have to go into the underwater tunnel to find Misty, Togepi and Bulbasaur. Ash and Tracey jump in and swim down the tunnel and reach the end, where the creature appears and attacks the companions! The SWAT team has come out of the sewer and reported back to the Mayor. They tell him they couldn't find a creature that matched the original description, but found 3 other creatures down there. The leader comes to the conclusion that the sewer could be infested with freaks like the ones they saw. The Mayor grows increasingly paranoid and orders that every manhole to be sealed and seal off the sewer. The group wakes up and sees an opening into the city. They also see Misty and Bulbasaur. Tracey found the creature, who actually is a giant Bulbasaur. He has been living down there for a long time and was actually using Vine Whip. Ash notices that it has the Mayor's symbol on its collar. Back at the Mayor's office in the City Hall, Nurse Joy explains to him why sending his SWAT team to seal all the sewer exits is a bad idea. She explains that Ash and his friends are in the sewers and they'll be trapped there if all the exits are sealed. The Mayor assures Joy that he will remove all the concrete as soon as the election is over. Later, while Mayor is taking a bath, an earthquake starts, then water forcefully erupts from the floor right under the Mayor's bathtub. The bathroom, the City Hall and the entire town are flooded. Ash asks if the giant Bulbasaur looked familiar to the Mayor. He sees his trademark "M" initial on Bulbasaur's collar and explained that he abandoned his Bulbasaur in the sewer because it wouldn't evolve. Upon learning this, the Mayor's Bulbasaur grabs him with Vine Whip. Misty and Tracey tell him how dare he abandon it, yet it's happy to see him. Many people are seeing the Mayor half-naked and he becomes very embarrassed. Ash and friends order him to apologize to his Pokémon for what he did. The SWAT team arrives, and the Mayor orders them to open fire on Bulbasaur. However, Ash steps in and commands Pikachu to use Thunderbolt on them and his Bulbasaur to use Vine Whip on them, which sends the SWAT team flying off. The Mayor says that he will offer a truce with his Bulbasaur, but Bulbasaur spins him around and throws him into his statue, toppling it and his chances in next week's election. Nurse Joy says that she will take care of the giant Bulbasaur at the Pokémon Center. Joy points to Trovita Island, which is almost far away. Meanwhile, Team Rocket has found an exit from the sewer system, but it has bars on it and they realize they are trapped. Jessie sneezes and breaks the metal bars, setting them free.