This story begins (as do many others) with the Five taking their summer holidays at Kirrin Bay.
Uncle Quentin has been working closely with a colleague, an American, who fears for the safety of his daughter after a
kidnapping threat. It is decided that his daughter, Berta, comes and joins the Five to hide away.
Berta has to disguise herself and, with Aunt Fanny's help, is transformed into "Leslie" a young, blonde haired boy!
Uncle Quentin and Aunt Fanny have to leave suddenly and the Five - with "Leslie" - are left in the care of Joanna the cook. It soon becomes clear that "Leslie" is being watched and the Five are put on their guard!
One night, George goes outside to the kennel with Sally (Berta/Leslie's dog) and is mistaken for the young American and kidnapped. The Five insist on finding her (against the
advice of the Police) but George had been smart enough to leave a few clues for the others to follow.
The trail leads to a fairground - and to Jo! (Their friend from Five Fall Into Adventure and Five Have A Wonderful Time.)
The race is on to rescue George before the kidnappers
discover they have the wrong child and give Uncle Quentin and his colleagues time to finish their work!