From the Super Mario Wiki
When the player decides to stop here, they will meet Bowser and his cohort, Koopa Kid. Their quest, Bowser: Accused!, is a mystery, where Shroomlock has accused Bowser of stealing a valuable painting. The player must help Shroomlock find a flaw in Bowser's explanation that will prove he's guilty to solve the crime. After that, he'll challenge them to the Bowser minigame Mush Rush. If the player succeeds at defeating the set number of Koopa Kids, Bowser will reward them with the Gaddget, Lip Sync.