The Five eat heaps of Joanna, the cook, and Aunt Fanny's delicious food. George generously plans to explore their very own Kirrin Island with the cousins after their exciting adventures the previous summer. However their plans are spoilt, first when Joanna is temporarily replaced by the reprehensible Mrs Stick, her husband Mr Stick and their ghastly son Edgar. Then Aunt Fanny is taken suddenly ill and has to go with Uncle Quentin to be treated in a far off hospital. The Five are left in the dubious care of the Sticks. As the Sticks were too horrid to live with, the Five decided to go camping at Kirrin Island and take care of themselves.

When the Sticks try to poison George's dog Timmy, George can take it no more. She hatches a secret plan to run away to Kirrin Island at midnight [brave little girl], thinking that her cousins will not agree. Julian, becoming suspicious of George's actions, convinces her that they should all run away to the island and pretend they have gone back to Julian's home in order to fool the Sticks.

While on the island, they lived there peacefully and discovered new things. And they find some new things some amazing things and thought that there are smugglers there, at Kirrin. A mysterious ship, a scream in the night and the discovery of a young girl's clothes alert the Five that there could be something sinister on the island. They later discover that the Sticks have kidnapped a little girl, the daughter of a very rich man, and with the girl they go to the police who are amazed to see the girl "the whole country is looking for!!" The police accompany them back to the island in time to trap and arrest the Sticks. They are not the masterminds behind the crime, they are being employed by the owner of ship, the Rover, and have been asked to keep her for a week for £20000, as the full ransom is £100000.