Julian, Dick, Anne, George and Timmy the Dog make up the Famous Five. Julian, Dick and Anne are brothers and sister, In the first book, Five on a Treasure Island, they meet their cousin Georgina. Georgina likes to act like a boy and hence is called George. Her dog, Timmy, is the fifth member of the group.

In FIVE RUN AWAY TOGETHER, 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. When Mrs Stick tries to poison George's dog Timmy, George hatches a secret plan to run away to Kirrin Island. When Julian catches her leaving, she decides to allow the other children to come, too.

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. The children discover the Sticks have imprisoned a young girl, Jennifer Armstrong, on the island and are holding her for ransom. After tormenting the Sticks into a retreat, the children rescue the girl and take her to the police, who are amazed to see the child "the whole country is looking for." The police accompany them back to the island in time to trap and arrest the Sticks. The kidnapped girl's father allows her to spend a week with her new friends on Kirrin Island.