To her horror the boy pulled out his hanky and began to sob into it - but they were queer sobs. Pip suddenly snatched away the boy’s hanky and stared at him. There wasn’t a single tear in his eyes - and he was laughing, not crying!

“Oh!” said this amazing boy, “oh, you’ll be the death of me! I can’t keep it up any more! Oh, Bets, oh, Pip, I shall crack my sides with laughing!”

It was Fatty’s voice! Fatty’s voice! Bets and Pip stared in the utmost amazement. How could this boy talk with Fatty’s voice?

The boy suddenly put his hand to his mouth and whipped out the curiously jutting teeth! With a quick look round to make sure no one was looking, he lifted his curly hair - and underneath the wig was Fatty’s own smooth hair!

“Fatty! Oh, Fatty! It’s you!” cried Bets, too astonished even to hug him.

“Golly, Fatty! You’re a marvel,” said Pip in awe. “You absolutely took us in. How did you get such a pale face? And those teeth - they’re marvellous! Your voice too - you talked just like a silly, shy French boy - and to think I tried to talk French to you too!”

“I know! The hardest thing for me was trying not to laugh,” said Fatty. “I did burst out just before your mother came into the room this morning, and I had to pretend I was howling. I say - didn’t I take you all in!”

“How did you dare to go and face old Clear-Orf like that?” said Pip. “However did you dare?”

“Well, I thought if I would deceive you as easily as all that, Clear-Orf would never, never guess,” said Fatty, walking on with them. “Come on - let’s go to Larry’s and you can say I joined you on the way up. We’ll get another laugh. And then we’ll have to talk about old Clear-Orf and that letter. I hope to goodness he doesn’t know how to test for invisible writing. That wasn’t a very polite letter.”

They went in at Larry’s gate, walked in at the side door and up to Larry’s room. Larry and Daisy were there. They stared in horror when they saw the French boy again.

“He wants to come too,” said Pip, hoping he wouldn’t giggle. “He met us in the road.”

“They were so, so, so kind,” put in Fatty, and he bowed deeply again, this time to Daisy.

Bets exploded into a laugh. Pip gave her a nudge.

“I can’t help it, I can’t help it,” giggled Bets. “Don’t glare at me, Pip, I just can’t help it.”

“What can’t she help?” said Larry in astonishment. “Honestly, she’s potty too.”

Fatty spoke suddenly in his own voice. “I hope you don’t mind me coming to tea, Larry and Daisy.”

Larry and Daisy jumped violently. It was so unexpected to hear Fatty’s voice coming from some one they thought was a queer French boy. Daisy gave a squeal.

“You wretch! It was you all the time! Fatty, you’re simply marvellous! Is that one of your disguises?”

“Yes,” said Fatty, and he took off his curly wig and showed it to them. They all tried it on in turns. It was amazing the way it altered them.

“The teeth are fine too,” said Larry. “Let’s rinse them and I’ll put them on. I bet you won’t know me!”

They didn’t! It made Larry look completely different to wear the odd, jutting-out teeth. They were not solid teeth, but were made of white celluloid, with pink celluloid above to make them look as if they grew from the gum.

“And your limp - and your voice! They were both awfully good,” said Pip admiringly. “Fatty, you took Mother in completely, too - it wasn’t only your disguise - it was your acting as well.”

“Oh, well - I was always good at acting,” said Fatty, in a modest kind of voice. “I always get the chief part in the school plays, you know. Before I decided to be a detective I thought I’d be an actor.”

For once the four children did not stop Fatty’s boasting. They all gazed at him with such rapt, admiring attention that Fatty began to feel quite uncomfortable.

“I think you’re wonderful,” said Bets. “I couldn’t possibly act like that. I should be scared. Fatty, how dared you go and face old Clear-Orf - and give him that letter too!”

“I think that was a bit of a mistake now,” said Fatty, considering. “If he does run a warm iron over the blank sheet, he’ll read the letter - and it’s a bit rude, really.”

“Awfully rude,” said Daisy. “I only hope he won’t go and show it to our parents. That really would be sickening.”

Pip felt alarmed. His mother and father were strict, and would not allow rudeness or bad behaviour of any sort if they could help it.

“Golly!” said Pip, “this is awful. I wish we could get the letter back.”

Fatty, looking like himself now that he had taken off the wig and the teeth, looked at Pip for a moment. “That’s a good idea of yours, Pip,” he said. “We will get it back. Otherwise he’ll certainly show it round to all our parents and we’ll get into a row.”

“I don’t see how in the world we can possibly get it back,” said Larry.

“What about one of us putting on a disguise, and -” began Fatty. But they all interrupted him.

“No! I’m not going to face old Clear-Orf now!”

“I wouldn’t dare!”

“Golly - he’d arrest us!”

“He’d see right through any disguise I wore!”

“All right, all right,” said Fatty. “I’ll go and face old Clear-Orf - in my French-boy disguise again - and I bet I’ll get that letter back too.”

“Fatty - you’re marvellous!” said everyone together, and Fatty tried in vain to look properly modest.

“How can you possibly get our letter back, though?” asked Larry. “I mean - old Clear-Orf isn’t likely to hand it meekly to you, is he?”

“Fortune favours the bold,” said Fatty. “I propose to be bold. First of all, I want to write another letter in invisible writing. Hand me an orange, Larry.”

Larry gave him an orange and he squeezed juice from it into a cup. Then he took out his pen, with its clean nib, got a sheet of white notepaper just like the one he had written on before, and began to write:

“DEAR CLEAR-ORF, - I suppose you think you will solve the next mystery first. Well, as your brains are first class, you probably will. Good luck to you! From your five admirers,

“THE FIVE FIND-OUTERS (AND DOG).”

Fatty read it out loud as he wrote. The others laughed. “There!” said Fatty, “if I can possibly exchange this letter for the other one, it won’t matter a bit if he goes parading round showing it to our parents!”

He stuck his teeth back under his upper lip, and at once his face altered out of all knowledge. Then he carefully fitted on the curly wig. It was a beauty.

“What else did you buy?” asked Larry.

“Not much, after all,” said Fatty. “The things were much more expensive than I thought they’d be. This wig took nearly all my money! I got these teeth, and two or three pairs of different eyebrows, some make-up paint that gives you a pale skin, or a red one, or whatever you like - and that foreign-looking cap. I got a cheaper wig too, which I’ll show you - mousy hair, and straight.”

He put on the foreign-looking cap and stuck it out at an absurd angle. Nobody would have thought he was Fatty. He began to limp across the room.

“Adieu!” he said. “Adieu, mes enfants!”

“He means ‘Good bye, my children,’ ” Pip explained to Bets, who watched with admiring eyes whilst Fatty limped along the passage to the head of the stairs.

“Good-bye, Napoleon!” called Bets, and every one giggled.

“I hope old Clear-Orf won’t get him,” said Larry. “He’s frightfully brave and bold, and awfully clever at this sort of thing - but Clear-Orf doesn’t like jokes played on him.”

“I wonder if Clear-Orf has been able to read the invisible writing yet,” said Bets. “I bet he was angry if he has!”

Clear-Orf was angry. In fact, he was almost bursting with fury. He had heated an iron, knowing that heat was one of the things that made most invisible writing show up plainly - and he had carefully ironed the sheet of notepaper.