This is the sixth novel in The Five Find-Outers mystery series. It is noteworthy among fans of the series as it marks the first appearance of Ern Goon.
The story starts with the younger Find-Outers quite excited that Fatty is arriving by train that morning. When Fatty does not disembark, they quickly realize that he is testing them -- he has surely dressed up in one of his famous disguises, and is trying to sneak by them! A quick inspection of the passengers finds a likely suspect; a boy of about the right build (that is, fat). They run after him, shouting "Hey, Fatty!", only to find that he isn't Fatty at all -- in fact, they have just insulted the nephew of their arch-nemesis, the village policeman, Mr. Goon.