In this installment of the Five Find-Outers series by Enid Blyton, Fatty assumes the role of the group's leader, showcasing his mastery of lock-picking, invisible ink, and clever disguises. The Find-Outers indulge in the excitement of their newfound skills, particularly the art of disguise. However, their adventure takes an unexpected turn when Pip's escapade leads him to an abandoned house with a peculiar secret room that defies explanation. Determined to uncover the truth, the children embark on a quest to unveil the owner of Milton House and unravel the mysteries concealed within its walls. As they dig deeper, they encounter the enigmatic "John Henry Smith" and communicate a dire warning. Fatty's courage leads him into the heart of danger as he infiltrates the house, discovering a cryptic notebook written in code.