One summer Janey, who is blind, and her elder brother Tim visit the island with their naturalist father who wants to find a rare orchid that might grow there.

Another English tourist, Mr Jones, arrives at the same time and also stays in the hotel (a village inn with a few rooms).

They are threatened by the crooks calling themselves Jones and Smith (not their real names) who had stolen many valuable jewels from deposit boxes during a bank robbery a few years before and hidden them in the island's sea-caves, but eventually the children are unharmed and the thieves don't manage to get the stolen treasure back.

As Perdita is now something of a hero the local children stop being so mean to her; also she is expected to start attending school soon after the end of the story and will be less isolated.

The 1971 BBC adaptation was a five-episode mini-series starring Fiona Kennedy as Perdita, Gillian Bailey as Janey and Spencer Banks as Tim.