Spy Classroom, Vol. 9 (light novel)

After the fierce battles in Fend, the members of Lamplight were granted a fourteen-day vacation. But as the girls are enjoying their holiday on a remote island, Klaus issues a mysterious order: "Until the day before the last, no one is to gather together." And on the final day, Annette vanishes. Between the island´s legend of pirates, a cursed murder case, and a girl claiming to be Klaus´s fiancée, the members of Lamplight work together to piece together the memories of their scattered thirteen days to locate their missing comrade.