From the Super Mario Wiki
Shroom City is a large city located on an unknown island, which also consists of a city built on the islands below. It is in the game Mario Party Advance.
After the player finishes making their passport, they meet Toad. Toad was telling the player and asks them why they are in this wonderful place. Then suddenly, Bowser and his little companion, Koopa Kid drops by and squishes Toad to crash the main place. Bowser crushes the main place of the Party World, dropping off Gaddgets and minigames to Shroom City, making them scattered all over the place. The player has to restore the main place by collecting Gaddgets and minigames from the locals in town.
In the game, there are fifty quests for the player to solve in order to gather a Gaddget from a citizen or from a Minigame from Bowser and the Koopa Kid. Some quests the locals have are easy, but many of them are hard along the way. Additionally, some quests include two characters named Shroomlock and Mr. E that appear during the quest to solve a mystery in the scene, although Shroomlock's quests are more like crime mysteries than Mr. E's. Shroomlock works in a organization called the Toadland Yard, while Mr. E explores Shroom City looking for mysteries that interest him.
When the player completes the first quest, the game goes to the Pipe House, where Bowser appears to shake the place to drop Bowser Head rocks that block the pipes that lead to Bowser's hidden locations. If the player completed a certain number of Quests, Bowser will break a Bowser Head rock for the player to enter to solve one of Bowser's quests.
Once all the citizens' quests are complete, the player can go to the Pipe House in the Desert Area to confront Bowser and Koopa Kid at the showdown.