Before The GameEdit
In the game's intro, Princess Peach is seen on a holiday in a town known as Rogueport. Finally getting a chance to be on her own and away from Toadsworth watching over her, she comes across a merchant who tells her to buy something. As Peach being interested in a box, what she described as "pretty", the merchant tells her that the box contains a magical map that leads to where a legendary treasure sleeps and a magic lock attached to it can only come off if someone with a pure and noble heart has it in their possession. Which means not even the merchant himself is able to remove the lock. The merchant has a feeling it might open for Peach and will allow her to keep whatever is hidden inside. Peach happily interested, tries and the box accepts her, thus creating a bright light and the game begins.
Before the adventure begins, Parakarry from the previous title is seen delivering a letter to the Mario Bros. residence. Luigi, who comes out of the house, checks the mail and notices the letter was delivered directly from the Princess and decides to read it to his brother, Mario. Along with the letter, came the mystical map that was found inside. Peach's letter stated that she picked up the map from a merchant in a town called Rogueport and told Mario to meet up with her there. There, they would begin their search for the treasure the map leads to. Her letter appears to be true as Luigi notices and shows Mario the mystical map she was talking about. Mario's adventure begins as he set sails to this new area. Upon his arrival to Rogueport, Peach is no where to be found.
During the GameEdit
Throughout the game, Mario would use the map to find all locations of the seven of the Crystal Stars. He would hold the map up to the Thousand Year Door to get the next location where the star is located. However, upon reaching Chapter 8 and acquiring all seven Crystal Stars, the map is no longer of use.
As Mario progresses through the game, Bowser (on his own with Kammy Koopa) and the X-Nauts (with the Shadow Sirens tagged along) would try to take the map from him to find the locations of the Crystal Stars. However, their actions are unsuccessful.