Lucy-Ann, Jack, Dinah, and Philip spend the holidays in a cottage near a strange, deserted castle, and find themselves in the midst of another adventure, involving a torch flashing at night and men at work in the depths of the castle.

Preparing for a trip by plane with Bill, the four children inadvertently go into the wrong plane, which takes off under frightening circumstances -- and find themselves stranded in an isolated valley scarred by war, and at the mercy of sinister men looking for a long-lost treasure.