The third in the mystery series is where Fatty really begins to take detective work seriously. The focus of the first six chapters is Fatty spending all his pocket money on disguises: a couple of wigs, several pairs of stick-on eyebrows, false teeth, some makeup, and a cap. Naturally Fatty tries out a disguise on the other Find-Outers, and completely fools them with his "frightful French boy" act. In addition to the disguises, Fatty astounds his friends with a few simple detective tricks, such as how to get out of a locked room, and how to write letters with invisible ink. Naturally all this comes into play later...but before that, Fatty suggests that he really ought to be head of the Find-Outers instead of Larry. Larry rather sportingly accepts this, and Fatty takes charge.