"Five on a Secret Trail" is the fifteenth book in Enid Blyton's popular "Five" series. George and Anne find themselves camping alone near Kirrin Cottage when they witness mysterious flashing lights from a nearby derelict cottage. They befriend a peculiar young boy involved in an archaeological dig. When Julian and Dick return from France, the group realizes they are being targeted to leave the area. As they uncover the truth, the question remains: who is behind these attempts to scare them away, and why?