While they help set up an exhibition at Hosho Art Museum, a glass slipper on display goes missing. Anna and Mikuru look for clues, certain that it was the work of the Phantom Thieves' Guild.

Anna and Mikuru are on a school trip, but Goemon appears.

A desperate letter arrives at the CUREtto Detective Agency: "The island could disappear!" Leading Anna and Mikuru to take a boat to the isolated island. To solve the case, they must unravel the secret behind an old island song.