Spy Classroom Short Story Collection, Vol. 2 (light novel)

Before she dies, Hearth, the leader of the legendary spy team “Inferno,” bequeaths a key of unknown purpose to Thea, a young girl who saved her from the brink of despair. Years later, Thea finally discovers a box that the key can open. Inside, she finds Hearth’s last testament—but it’s missing a word. Soon, misunderstanding begets misunderstanding as everyone in Lamplight gets sucked into uncovering the mystery of Hearth’s note!