The Butt Detective solves crimes by literally detecting butts. That's the entire premise, delivered with zero irony or winking at the camera. This movie commits so completely to its absurd central conceit that the mystery plot becomes secondary to watching a children's show operate with the deadpan confidence of a noir detective thriller.

Koala asks Butt Detective to track down a missing relative who vanished from an idol contest, and the case leads him to Stardust Island, where the contest is held. The island is also home base for Phantom Thief Academy, an international criminal group led by Phantom Thief G, which is quietly pushing forward a secret scheme known as the Dark Age Plan. Butt Detective slips onto the island disguised as an idol. Along the way he teams up with Phantom Thief U, who wants to take back the precious Moonlight Stone from Phantom Thief G. The film also sheds new light on Brown's past, mixing comedy and mystery in a feature-length case.