Koala requests Butt Detective to locate a vanished relative who left an idol competition during the final night, with high stakes. The investigation leads Butt Detective to Stardust Island, the venue of the contest and also the base of the global crime syndicate Phantom Thief Academy, commanded by Phantom Thief G. There, the syndicate pushes forward its covert plot, dubbed the Dark Age Scheme. Butt Detective infiltrates Stardust Island posing as an idol contestant. Simultaneously, he allies with Phantom Thief U, who aims to recover the precious Moonlight Stone from Phantom Thief G.
