From the Super Mario Wiki
When the player decides to stop here, they 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 prove he is guilty to solve the crime. After that, he challenges them to the Bowser minigame Mush Rush. If the player succeeds at defeating the set number of Koopa Kids, Bowser rewards them with the Gaddget, Lip Sync.
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