On a quest to find fireflies, Julian, George, and Timmy wander through the forest in the middle of the night. There, they accidentally observe two men, one of whom suddenly disappears without a trace. In their search for him, they discover an underground hideout, but it cannot be found the next day. When Dick claims that Julian has made everything up, the matter needs to be resolved. Furthermore, there are things in Kirrin that have been missing for years. Even Uncle Quentin is affected by this. With persistence and courage, the Famous Five uncover several dark secrets.