The Mystery Menace (Japanese: ちかどうのかいぶつ！？ Monster of Underground Tunnel!?) is the 21st episode in Pokémon: Adventures on the Orange Islands season.
Ash and co. climb over a mountain and notice the city of Trovitopolis. Team Rocket tries (and fails once again) to trap our heroes, however they do fall into the Trovitopolis Sewer System. There they find a monster and who should save them but Officer Jenny, unfortunately they wake up in a jail cell. They were put there by the Mayor of Trovitopolis, even though they were only wandering around the sewers for Ash's Bulbasaur. What is the conspiracy the Mayor is trying to hide, and why would the sewers be off limits?
As the gang continues the journey, Ash asks if they need to eat if they are hungry, although Misty tells this is what he thinks all about. Tracey sees a big city and the heroes are pleased to arrive at a settlement. Team Rocket also appear and like that they are heading for the trap. Meowth knows if Pikachu is on a spot, the dynamite will blow up and Pikachu will fall in the trap. Meowth tells to prepare, so James and Jessie chant the motto. Meowth stops them, as they need to make this a surprise for the twerps. James pushes a lever and while no exploison is heard, Team Rocket fall down in their own trap. Misty heard the scream and while the gang does not know why would someone scream, a crack in the ground caused by Team Rocket spreads and the heroes fall down in the hole.
When Pikachu and Togepi wake Ash and Misty up, Ash and Misty pull Tracey out of rubble. Tracey thanks them, but Ash replies he does not need to thank him but stand up, as Tracey sits on him. Misty sees a tunnel and head through it. At the end of the tunnel, they arrive to the sewers. Tracey believes they need to find a manhole to get up. The heroes continue and something passes, as Ash hears a small bell. Misty and Tracey think it is a side effect of him falling down the hole. Ash gets tapped and tells Pikachu to stop it. However, Pikachu is in front of him, so they notice a monster. Ash goes to order Pikachu to use Thunderbolt, but Misty warns him they'll get shocked as well. Ash sends Bulbasaur, who uses Razor Leaf. The monster stops the Razor Leaf and takes Bulbasaur away, into the water. Ash goes to rescue Bulbasaur, but Tracey, Misty and Pikachu hold him back, and Tracey tells him that the monster would drag him down, too, but Ash doesn't care and tries to struggle out of Tracey, Misty and Pikachu's arms. Officer Jenny removes the manhole cover, hearing Ash's shouts, then lowers a rope so they can get up. Ash refuses to go until Bulbasaur is rescued, but Tracey, Misty and Pikachu tie Ash up and Officer Jenny pulls him out, with Pikachu beside him, and arrest him, along with his friends, from being in a sewer that is dangerous.
Later, the heroes are sent to jail, as Ash protests to be freed and help Bulbasaur. Jenny appears and unlocks the jail, telling there were strange things in the sewer. The mayor appears and tells Jenny to have them stay in jail for a week. Jenny refuses, as they have arrived today and have commited no crime. Ash only cares to free his Bulbasaur. The mayor allows them to go out, but they must not mention the monster. Tracey and Misty object, though the mayor tells it is for the public safety. Jenny tells the mayor they cannot keep the secret up, to which the mayor responds they can try. Jenny tells Ash, Misty and Tracey the monster was seen taking food. The mayor tells the monster needs to be captured, as the elections are next week. Misty sees there is no harm if it wants only food. Nurse Joy appears and tells the mayor he cannot do that. The mayor responds that she needs to run her Center and he needs to run the city.
Nurse Joy tells the mayor wants to eliminate the creature instead of capture it. The heroes criticise mayor's plan and Jenny tells the mayor he may do illegal activity. The mayor gets angered and yells he runs the city here. The heroes contact Oak, who agrees that this creature may be a new Pokémon or a Pokémon undergone a mutation. Nurse Joy knows it is not possible, as the mayor wants to be re-elected. The heroes see the mayor's campaign posters and statue and see a strange mark. Ash tells them they need to find Bulbasaur. The mayor orders his special police to get inside and eliminate the creature. The heroes see that it will be hard to get in, otherwise this creature will not be seen again. Nurse Joy points at the back end of the sewer. Ash goes to climb up, but falls down.
Team Rocket wandered the sewers and Meowth got hungry. He gets tapped by the creature and thanks Jessie and James, but tells it won't get rid of his hunger. Team Rocket notice the creature and flee the scene. Team Rocket are exhausted and hear footsteps, then see a flashing light. James sends Weezing, who uses Smokescreen (dimming the vision of the twerps, whom they did not see) and Team Rocket run off. After the smoke clears, the heroes heard Team Rocket. Misty sees Togepi is gone, who almost ran in water. Misty grabs it, but soon, the creature takes Togepi and Misty away. Ash knew they were coming in the sewer and sends Muk (who comes on him). Team Rocket are *very* exhausted and hear some footsteps. Meowth encourages Jessie and James to scare this monster and make fearsome (which are silly) faces. Team Rocket appear to the special police, who report to the mayor they see three creatures. The mayor is surprised, but orders to shoot, making Team Rocket run away from the goo they shoot.
Muk lead Tracey and Ash to a dead end. However, seeing a hidden tunnel, Tracey and Ash dive in water and go through the tunnel. At the end, Ash gets out and sees the monster. The police report they found three creatures and think the sewer is full of them. The mayor orders to block every manhole and seal the sewer. The police use concrete to barricade the manholes and sewer entrances. Ash wakes up and sees Bulbasaur, pleased he found it. Ash gets tapped and Tracey tells it is an overgrown Bulbasaur, who uses Vine Whip. Ash wonders how it got here. Tracey knows that someone left it, as it has the collar and the bell. Tracey believes it wanted company, which is why it took Ash's Bulbasaur and Togepi with it. Ash pats it and notices a mark on the bell.
Nurse Joy protests against mayor's decision. The mayor tells they did not exterminate the creature, but Joy reminds him the children are there. The mayor tells he'll let them out after the election is over, but Joy tells the elections end in next week. The mayor responds children shouldn't play in the sewers, making Joy annoyed. The mayor has a bath and knows he'll win the elections with the problem exterminated. Soon, he feels the ground shaking and a leak of water comes out of his tub, letting the water out as the heroes and the overgrown Bulbasaur float out of the mayor's mansion. The mayor sees the heroes and is asked if the Bulbasaur looks familiar. The mayor sees the Bulbasaur and the mark on the bell. The mayor remembers he left the Bulbasaur floating into the sewers because it wouldn't evolve. His Bulbasaur lifts him up, still glad to see him, while others aren't to hear this story.
The mayor asks Bulbasaur to put him down. Misty tells he should apologise first, to which the mayor objects and orders his special police to fire, telling Bulbasaur is taking him hostage. Pikachu uses Thunderbolt to stop them, while Bulbasaur uses Vine Whip to blast them off. The mayor's Bulbasaur spins him and blasts him off, making the mayor fall onto his own statue and crashing it. Later, Jenny tells with the elections coming up, there will be a new mayor soon. Joy tells she will be taking care of the overgrown Bulbasaur. Ash asks if there is Trovita Island nearby. Joy points at an island and tells it will take them few minutes to get there. Jenny tells with a ferry around, they can use the boat and go there. Meanwhile, Team Rocket are in the sewers and see daylight, but the way is barricaded. Jessie sneezes and the bars fall down and they are happy they are out.
The "Who's that Pokémon?" of this episode is Zapdos.