Kirrin Island belonged to George: it was her very own, and she felt aggrieved when her father 'borrowed' it for one of his experiments. And to make matters worse, he built a mysterious tower in the ruins of the old castle.

The Five couldn't really understand Uncle Quentin's wish to be left quite alone, and as it turned out, it was a good thing that George took matters into her own hands one night—otherwise there would have been no more adventures on Kirrin Island, and Timmy would never again have been able to prove how intelligent he was.

This is the Five's sixth adventure.