He could hardly believe his eyes when he read the faint brown letters! He swallowed hard, and his froggy eyes almost fell out of his head.

“All right. We’ll see what your parents say to this!” said Mr. Goon, speaking as if the children were there in front of him. “Yes, and the Inspector too! This’ll open his eyes, this will. Rude, cheeky toads. No respect for the law! Ho, now I’ve got you! You didn’t think as I’d be smart enough to read your silly invisible writing, did you?”

Mr. Goon had several things to do that day, and it was not until the afternoon that he decided to go and display the letter to the children’s parents.

“Don’t wonder they dursent come and deliver the note themselves!” he thought, remembering the queer boy who had delivered it. “Got some friend of theirs, I suppose. Staying with one of them, I’ll be bound.”

He decided to go to the Hiltons first. He knew how strict Mr. and Mrs. Hilton were with Pip and Bets.

“Open their eyes nicely, this will,” he thought, trudging off. “Hallo! - there’s that little Frenchy fellow. I’ll just find out where he’s staying.”

“Hi!” yelled Mr. Goon to Fatty, who was sauntering along on the other side of the street, hoping that the policeman would see him. “You come here a minute.”

“You call me?” said Fatty politely, in the high, foreign kind of voice he had used before.

“I got a few questions to ask you,” said Clear-Orf. “Who gave you that there rude note to deliver to me this morning?”

“Rude? Ah, non, non, non - surely it was not rude!” said Fatty in a shocked tone, wagging his hands just as his French master did at school. “That I cannot believe, Mr. Poleeeceman.”

“Well, you look here at this,” said Mr. Goon. “Maybe you can tell me whose writing this is, see?”

He took the envelope from his pocket, and pulled out the sheet of paper. “There you are - you take a squint at that and tell me if you know who wrote that rude letter.”

Fatty took it - and at that moment the wind most conveniently puffed down the street. Fatty let go the paper and it fluttered away. Fatty sprinted after it at once, and, when he bent down to pick it up, it was easy to slip it into his pocket and turn to Clear-Orf with the other letter in his hand.

“Drat it, it nearly went!” said Mr. Goon, and he almost snatched it from Fatty’s hand. “Better not flap it about in the wind. I’ll put it back into the envelope.”

He did, and Fatty grinned to himself. It had been so easy - much, much easier than he had expected. What a kind puff of wind that had been!

“Where are you walking to, Mr. Poleeeceman?” asked Fatty politely.

“I’m going down to Mr. and Mrs. Hilton,” said Mr. Goon righteously.

“Then we part,” said Fatty. “Adieu, dear Mr. Poleeeceman.”

He went off round a corner, and Mr. Goon stared after him. He felt puzzled. but he didn’t know why. “That French boy isn’t half queer,” he thought. He would have thought him queerer still if he had seen what Fatty did round the corner!

Fatty pulled off his wig, took out his teeth, removed his queer-looking cap, and took off the rather gaudy scarf he wore. He hid them all in a bush.

Then, looking once more like Frederick Algernon Trotteville, he hastened to the house where Pip and Bets lived, and where Mr. Goon had already gone. He went in and gave the usual call for Pip, although he knew quite well he wasn’t there, but was at Larry’s.

“Oh, there you are, Frederick,” said Mrs. Hilton, looking out of the door of the sitting-room. “Come here a minute, will you? Pip is out, and so is Bets. Mr. Goon is here with a very extraordinary story. Apparently he thinks that you and the others have been guilty of most unnecessary rudeness.”

“How extraordinary!” said Fatty, and went into the sitting-room. He saw Mr. Hilton there too, and Mr. Goon sitting on a chair, his knees turned out widely, his great hands flat on them.

“Ho!” he said, when Fatty went in. “Here’s one of them what wrote that invisible letter. Now, ma’am, I’ll just show it to you, and you’ll be able to read it. Talks about my brains creaking for want of oil!”

Mr. Goon took out the sheet of paper from the envelope and laid it on the table. It was blank, because the writing had not been warmed up. Mr. Goon looked at it, and was annoyed. The lettering had been there last time he had looked at it.

“It wants a hot iron again,” he said, much to Mrs. Hilton’s surprise. “Could I trouble you to procure me a hot iron, ma’am?”

One was warmed and then Mr. Goon ran it over the sheet. “There you are!” he said in triumph, as the faint brown lettering became visible, “you just read that, ma’am and sir - what do you think of that for a letter sent to a reper - er - representative of the Law!”

Mrs. Hilton read it out loud:

“ ’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).’ ”

There was silence. Mr. Goon’s eyes bulged. This was not what he had read before! He snatched the letter.

“Well, Mr. Goon,” said Mr. Hilton, entering into the matter suddenly, “I can’t see what you have to complain about in that. Quite a nice, complimentary letter, I think.

Nothing about your brains er - er - creaking and wanting oiling. I don’t understand what you are complaining of.”

Mr. Goon read the letter again hurriedly. He couldn’t believe what he saw! “This here ain’t the letter,” he said. “There’s some dirty work going on. Did you write this letter, Master Frederick?”

“I did,” said Fatty, “and I can’t think why you should object to us expressing our admiration for you - or perhaps you think you haven’t got first-class brains?”

“That will do, Frederick,” said Mrs. Hilton.

Fatty looked hurt.

“What’s become of the letter I first had?” said Mr. Goon, feeling more and more puzzled. “Yes, and what I want to know is - are you children messing about with any more mysteries? Because if you are, you’d better tell me, see? If you go snooping around trying to find out things, you may get into Serious Trouble.”

Fatty couldn’t resist the temptation to let Clear-Orf think he and the other children really were trying to solve another mystery. So he looked very solemn indeed.

“I can’t give any secrets away, Mr. Goon, can I? It wouldn’t be fair.”

Mr. Goon at once thought there must be a secret, a mystery he didn’t know about. He got so red in the face that Fatty thought it was about time he was going.

“Well, I must be off,” he said to Mrs. Hilton, in his politest voice. “Good-bye!”

And before Mr. Goon could think of any good reason for stopping him, he went! He exploded into loud laughs as soon as he was out of earshot. Then he decided he had better go and get his disguise from the bush. He would put it on again to save carrying it, and would pop back to his house to fetch old Buster.

So, in a few minutes Fatty, once more in disguise, was walking home looking the same curly-haired, queer, rabbit-toothed boy that Mr. Goon had already seen twice that day.

And Mr. Goon spotted him just as he walked in at his gate! “Ho!” said Mr. Goon, pleased, “so that’s where that little varmit is staying - with that Frederick Trotteville! I’ll be bound he had something to do with altering that there invisible letter - though how it was done beats me! I’ll just go and make a few inquiries there, and frighten the life out of that Frenchy fellow.”

So, to Mrs. Trotteville’s enormous surprise, Mr. Goon was announced and came ponderously into her drawing-room.

“Good afternoon, ma’am,” said Mr. Goon. “I just came to ask a few questions of that foreign boy you’ve got here.”

Mrs. Trotteville looked as if she thought Mr. Goon had gone mad. “What boy?” she said. “We’ve got no foreign boy here at all. There’s only my son, Frederick.”