Julian, Dick, and Anne are going to spend their vacation at Kirrin Cottage with their cousin, Georgina, whom they've never met. Georgina is first very upset about having to spend her time together with her cousins. But after a while the children become friends. Georgina (George) brings her cousins to Kirrin Island. They get caught in a storm. The waves threw up a big shipwreck. When the children explore the wreck they find a box with a map. A map to find ""indigo"". What is ""indigo"", if not.... GOLD! But the children are not alone looking for the gold....
The Famous Five go on a camping trip with Julian and Dick's science teacher Mr Luffy. Everthing seems normal but when the Famous Five are involved, nothing is normal. Spook-trains in the dead of night! They seem to vanish into thin air-where do they go? The Five follow the tracks-and discover a very unusual, underground destination.
Poor Kirren Cottage!!! A big tree has fallen in on the cottage in the middle of the night, as the five have just got there Aunt Fanny despairs. But Uncle Quentin comes up with a plan. A proffesor friend and his son where meant to be coming but as the cottage is no longer liveable Quentin suggests that the Five stay at his house, Smuggler's Top!!! Are there still smugglers there? When the Five get there they make some surprising discoveries including someone signalling out to sea, hiding places and underground tunnels.