The game opens at the Mushroom Kingdom when Princess Peach, Mario, Luigi, Toad and Toadsworth are flown to Pi'illo Island. There, they meet Broque Monsieur and Starlow, both returning from the previous game in the series, Bowser's Inside Story. Soon after they arrive on the island, the gang gets introduced to Antasma, the bat king, who has teamed up with Bowser. They seek to acquire the Dream Stone, a powerful artifact that grants its user endless wishes. Together, Mario and Luigi must defeat him to save the island while battling enemies and traveling through Luigi's dreams along the way. In the Dream World, Mario meets "Dreamy Luigi", Luigi's Dream World counterpart, who helps Mario navigate and battle the Dream World. They also encounter Prince Dreambert, the leader of the Pi'llos, who helps the heroes in their quest. Once Mario and Luigi make it to Bowser and Antasma, Bowser reveals he'd been using Antasma the whole time, and takes the Dream Stone, leaving Antasma to be destroyed by Mario and Luigi. The Dream Stone is soon destroyed by Peach and Starlow, only for its fragments to be inhaled by Bowser, who transforms into an empowered living nightmare of himself called "Dreamy Bowser". Mario and Luigi defeat Bowser, who turns back to normal, rescue Peach, and finally return to their vacation on Pi'llo Island.