Koala asks Butt Detective to find a missing relative who disappeared from an idol contest. The case takes Butt Detective to Stardust Island, the site of the contest, and also the home of the international criminal organization Phantom Thief Academy, headed by Phantom Thief G. Here, the organization advances its secret conspiracy, codenamed the Dark Age Plan. Butt Detective enters Stardust Island under the guise of an idol. At the same time, he joins forces with Phantom Thief U, who is out to steal back the valuable Moonlight Stone from Phantom Thief G.