About half-past two that afternoon Pip and Bets set off to go to Larry’s. They had to go through the village, and to their horror they saw the French boy limping along the street.

“Look! there’s that awful boy again,” said Pip. “We’ll just grin at him and go on. Don’t let’s stop, for goodness’ sake, Bets. He might start jabbering at us again, or howling into his hanky.”

The boy went in at a gate. It was Mr. Goon - the policeman’s - gate. He had a note in his hand.

“Look! I bet Fatty has got his Frenchy friend to deliver that invisible letter!” said Pip. “Let’s just wait and see what happens. He’s knocked at the door, so old Clear-Orf may open it.”

The two waited near the gate, half-hidden by a bush. They saw the door open, and Mr. Goon’s red face appeared.

“I have zumsing for you,” said the boy in a foreign accent. “Mistaire Goon, is it not?”

“Yes,” said Mr. Goon, looking in surprise at the boy. He never remembered having seen him before. The boy presented him with a letter, bowed deeply and courteously, and waited.

“What are you waiting for?” said Mr. Goon.

“I not understand,” said the boy politely.

Mr. Goon appeared to think the boy was deaf. So he raised his voice and shouted. “I said - what you waiting for?”

“I wait for a - what you say? - answer. Ah, yes, I wait for the answer,” said the boy.

“H’m!” said Mr. Goon, and slit the envelope open. He unfolded the blank sheet and stared at it. His face went purple.

“See here!” said Mr. Goon, and he thrust the blank letter in the boy’s face. “Some one’s been playing a joke on me - silly sort of joke, too - wasting the time of the Law like this. Who gave you the letter?”

“I not understand,” said the boy, and smiled politely at the policeman, showing all his jutting-out teeth. “It is a mystery, is it not? A letter with nothing in it. Ah, truly a great mystery!”

The word “mystery” seemed to strike Mr. Goon. Since the children had solved two strange mysteries before he did, he had been rather sensitive about mysteries, and terribly afraid that the children might happen on a third one before he did. He gazed at the letter.

“Maybe it’s a secret letter,” he said. “Maybe it’s got a secret message. Who gave this to you, boy?”

“I not understand,” said the boy irritatingly.

“Well - I’ll test the paper for secret ink,” said Mr. Goon most surprisingly.

Bets gave a gasp. “Oh, Pip!” she said in a whisper. “It’s got such a rude message!”

The boy seemed to think it was time to go. He raised his cap, bowed deeply once more, and limped down the path, almost bumping into Bets and Pip.

“Bon jour,” he said courteously. Bets knew that meant good-day. She hardly dared to answer, because she was so afraid she might make him burst into tears again. Pip nodded curtly to the boy, took Bets by the arm, and moved smartly up the street.

To their annoyance the boy followed. “You will take me to tea with your friends?” he said, to their great horror.

“Certainly not,” said Pip, getting annoyed. “You can’t ask yourself out to places like that.”

“Ah, thank you a million times. You are so kind,” said the boy, and walked with them.

“I said, no, we can’t take you,” said Pip. “Go home.”

“I come, I come,” said the irritating boy, and linked his arm in Pip’s. “You are so, so kind!”

“Goodness, what are we to do with him?” said Bets. “I bet Fatty told him to come and meet us and ask to go with us. Fatty would be sure to want to get rid of him. He’s awful.” She turned to the boy.

“Go home,” she said. “Oh dear, I feel as if I’m talking to Buster when I say that! Do go home!”

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.