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.

Eriza Kurusu appears before Anna and Mikuru and tell thems, "I want to join the CUREtto Detective Agency!"

Shiruku Ieiri invites Anna and Mikuru to her show, and when they come to her dressing room, she shows them a calling card that was left by the Phantom Thieves' Guild. As they search the theater before the performance, Anna spots Luluka.