After defeating Blizzard Midbus, Bowser goes on to free the Dark Star. However, it goes mad and escapes into Bowser's body. Starlow asks what's wrong and Bowser says his chest is burning and starts to suck in cold air that is coming out of the defeated Minbus' nose. Starlow urges Mario and Luigi to follow the Dark Star into the Airway which is now accessible thanks to the now frozen platforms. Mario and Luigi endure a wild goose chase going after the Dark Star who is eating up Bowser's DNA slowly turning into Dark Bowser. Mario and Luigi eventually corner the Dark Star and they all get into a battle. Mario and Luigi defeats the Dark Star but it escapes Bowser's body and fully turns into Dark Bowser. Bowser decides to chase after it in hot pursuit.