Julian, Dick and Anne arrive in Kirrin Cottage to stay with George (real name Georgina) for the holidays. They plan to spend time exploring Kirrin Island but their happiness is spoilt when Aunt Fanny falls ill and has to leave with Uncle Quentin to be treated in a far-off hospital. They are cared for by Aunt Fanny's temporary cook, Mrs Stick, who is accompanied by her husband and their ghastly son Edgar. The Sticks and the four children come to hate each other. Mrs Stick repeatedly tries to poison George's dog Timmy, prompting George to hatch a secret plan to run away to Kirrin Island. When Julian catches her leaving, she decides to allow the other children to go with her.
The children find evidence of other people visiting the island and suspect smugglers. The discovery of a young girl's toys and clothes point to something sinister going on.