The game switches between three characters. Mario and Luigi are found together inside Bowser for the first part of the game, exploring a 2-D platform-style environment. However, battles are still turn based, and are initiated by touching enemies. Bowser explores the Mushroom Kingdom in a 3/4 overhead-view world, much like previous Mario & Luigi games. After a certain point in the game, Mario & Luigi can go outside of Bowser's body to explore the overworld as well. They can then use pipes to return to the inside of Bowser's body. - Wikipedia
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