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 must go to a far-off hospital. They are cared for by Aunt Fanny's temporary cook, Mrs. Stick, who repeatedly tries to poison George's dog. George hatches a plan for all the children to run away to Kirrin Island. While there, the children find evidence of other people visiting the island, and they suspect smugglers. The discovery of a young girl's toys and clothes point to something sinister going on.