Tip: Make sure that ALL exit-enabled pipes, doors, etc. have a screen exit defined, or they'll take you to Level 0, the "endless" bonus game. Unless Start+Select is enabled, the only way out of it is to reset the game.
Welcome to the Fungus Islands! Bowser has given up trying to kidnap Princess Peach (for now), and his son, Bowser Jr, has set his sights on the Fungus Islands. Mario and Luigi, being on vacation there, naturally, have to stop Bowser Jr's plans. Can the brothers stop the Colonization of the Islands?