The sixth book in the series once again sees the Five taking their school holidays at Kirrin Bay.
George is furious that her father had set himself up on Kirrin Island in order to carry out some top secret work. In addition to that, he had erected a huge silver tower that stood higher than Kirrin Castle! After a short visit to the island, they learn that Uncle Quentin is living in the dungeons but they can't fathom out how he gets in and out. The only known entrance is shut tight with weeds growing around it!
The Five befriend a new local boy, Martin Curton, who had moved into a cliff-top property with his father. George takes a dislike to Martin as she feels he asks too many questions about Kirrin Island and her father's work.
Uncle Quentin suspects that someone else is on the island with him - a spy perhaps! - and asks to "borrow" Timmy for protection. George has to make a hard decision but agrees to let Timmy stay on the condition that her father signals every night from the tower and takes Timmy so she can see him.
George panics when she doesn't see Timmy and secretly sets off late at night to the island. Her suspicions turn out to be well founded, as foul play is well and truly afoot on the
Timmy surprises everyone by turning up back at Kirrin
Cottage. He hadn't swam and he hadn't come by boat! Then a trip to the old quarry yields some exiting results!