In the seaside town of Rogueport, Princess Peach acquires a mysterious treasure map said to point to a great fortune hidden behind the fabled Thousand-Year Door, sealed beneath ruins built atop a town that sank long ago. Peach sends the map to Mario, but she is soon kidnapped by an alien group called the X-Nauts who are after the same treasure. Mario travels to Rogueport and learns that the map reveals the locations of seven Crystal Stars needed to open the door. Gathering allies along the way, he sets out to collect the stars, rescue the princess, and uncover the secret of what lies behind the Thousand-Year Door.