Siblings Julian, Dick, and Anne have come to Kirrin to spend the remainder of a school holiday with their tomboy cousin, George, and her dog, Timmy. When a mysterious girl arrives at Kirrin Cottage in the middle of the night, George and her cousins are thrilled. Berta is an American girl trying to elude kidnappers, and the Kirrin Household has been asked to protect and disguise her. Berta cuts her hair, dons boys' clothes, and is transformed into the handsome Lesley — and the Five are on constantly lookout for potential kidnappers. Will the disguise be successful, or will Berta be recognised and find herself in danger?