The book starts off when Larry, Daisy, Fatty, Pip and Bets are having fun with new disguises that Fatty brought back from Morocco. While all the Find-Outers, except for Fatty are dressed up. Ern, and his two brothers, Sid and Perce come to visit the Find-Outers. He is shocked when he sees four foreign-looking people with Fatty. Fatty tells the three brothers that they are visiting and are related to/friends of Prince Bongahwah, a Prince who is staying at the same camp as Sid, Perce and Ern for the summer, and that Bets is Princess Bongawee. The "foreigners" speak broken English and their own whimsical made-up "language" (especially Bets). Mr Goon, the village bobby (otherwise known as 'Clear Orf') happens to see all of them when they go for a walk (Larry, Daisy, Pip and Bets still in disguise) and believes the same story Fatty has told to Sid, Pece and Ern about the four 'foreigners'.
Later in the Story, Prince Bongahwah is kidnapped and Goon tells Inspector Jenks that he has met relatives of Prince Bongahwah to help with the case, in Particular the prince's sister, 'Princess Bongawee' (Bets in disguise) but finds out he has been fooled by Fatty. Fatty gets into trouble after however, he is still allowed to look into the case. Soon Sid has an unusual discovery - as he is mad (or 'dippy' according to Ern) about babies, he pushes a pram with twins in it and finds the bottom of it false, below it a person with a dark face which Sid is very sure is the Prince. After finding a button in the Prince's sleeping bag, a few interviews and some false trails, the gang find themselves at another dead end.
After a few bits of luck, they find a baby show for twins at a fair, hoping to track down the twins in question. They also bump into Mr Goon who is also at the fair for the same reason. Him and the gang look at the all the twins in the show (Mr Goon however making all the babies cry) but couldn't find the right twins. After their disappointment, Fatty manages to bribe one of the fair people to wind up Mr Goon and manage to get him on an animal roundabout where he gets spun round and round, dizzy in front of the watching (and laughing) crowd.
On the bicycle ride home Pip sees a shirt hanging to dry on a washing line of one of the parked caravans with buttons identical to the one they found in the Prince's sleeping bag. They piece up that the people living in that caravan would know the whereabouts of the Prince and Fatty (disguised as a peddler called Jack Smith) goes there later to find out that one of the people living in the caravan pretended to be the Prince; in the first place and they switched him and the real prince before camp started so he was impersonating the prince the whole time through summer and sneaked out the night before it was reported that the Prince had been 'kidnapped' while the Prince had been gone the whole time taken hostage by the boys uncle. He also finds out where the real prince is and decides to go off to the place the next day. Fatty soon goes home, bumping into Mr Goon who suspects that Fatty (still in disguise) is a tramp with a stolen bike. He runs off, leaving his bike with Mr Goon who reports it. Fatty gets it back after calling Mr Goon who exaggerates the truth and leaves Goon giving the Princes whereabouts which Goon doesn't believe until he hears a report from the nearby police station confirming that Fatty's information wasn't made up. Hearing this, Mr Goon decides to go rescue the Prince before Fatty and the others do and is reported the next day that he disappeared.
Fatty hears the news from Inspector Jenks and decides to go find him along with the gang and Ern (as Perce has to go get rope for the tent and Sid switches from buying toffee to buying nougat). They soon get locked up in a room beyond the marshes and is therefore trapped but sees a room with barred windows at the other side of the building. Fatty uses his 'how to get out of a locked room' trick and slips out to the other room and finds the real Prince inside and drags him back to the room with the other five. The men soon find out that the Prince has supposedly escaped and think that the find-outers helped him so they do a search of the room, finding the prince in a cupboard. Straight after the Prince is found, a squad of police officers surround the building, including Inspector Jenks who arrests the men and rescues the Find-outers, Ern and the Prince who tries and tells his story.
When they think that the whole mystery is closed they realise that Goon is still missing and didn't see him in the building. An officer comes up to them and says there has been weird noises coming from a shed not far off the building. Goon comes out of the building mad at the fact that he was locked up and beaten again by the gang. Fatty however sticks up for him and it's a rare moment when Goon thinks that Fatty isn't so bad after all.