Perdita, an orphan, lives on a remote island in the Hebrides. Some say she has supernatural powers, including 'the second sight': and certainly there are things she has done - uncanny things - that seem to substantiate this.

Jones has arrived, uninvited, on the island and called on Mr Smith. Janey and Perdita have become friends, and Tim has found, in a cave, what he believes to be a ruby. That night Mrs Hoggart is startled by a scream from Janey's bedroom.

Mr Hoggart has been taken to hospital, and Smith has decided that Jones must return to the mainland. Tim's gem, found in the cave, has been stolen.

Tim is convinced that Smith and Jones are crooks. An old newspaper links Jones with a London jewel robbery. The children are surprised in the cave by Jones.

Jones has been arrested and Tim has told the police about Smith. Perdita, convinced of Smith's innocence, has warned him of Tim's action and that he must be prepared to clear his name.