When the excitement of Christmas was over, the Christmas trees taken down and re-planted in the garden, and the cards sent away to a children’s hospital, the children felt rather flat. Fatty apparently was staying at his grand-mother’s, for they saw nothing of him, and had a post-card saying, “Back soon. Fatty.”

“I wish he’d come back,” said Bets. “Suppose a mystery cropped up? We’d have to be Find-Outers again - and our chief wouldn’t be here.”

“Well, there isn’t any mystery,” said Pip.

“How do you know?” said Bets. “Old Clear-Orf might be trying to solve one we don’t know about.”

“Well, ask him then,” said Pip impatiently, for he was trying to read, and Bets kept interrupting him. He didn’t really mean Bets to go and ask the policeman, of course. But she couldn’t help thinking it was rather a good idea.

“Then we should know if there was going to be something for us to solve these hols,” thought the little girl. “I’m longing to hunt for clues again - and suspects - and track down things.”

So the next time she met the policeman she went up to him. “Mr. Goon, have you got a mystery to solve these holidays?” she asked.

The policeman frowned. He wondered if Bets and the others were on the track of something he didn’t know about - else why should Bets want to know if he was solving one?

“Are you interfering in anything again?” he asked sternly. “If you are, you stop it. See? I won’t have you children messing about in jobs that properly belong to me. Interfering with the Law!”

“We’re not interfering or messing about,” said Bets, rather alarmed.

“Well, you clear orf,” said Mr. Goon. “You’ve put a spoke in my wheel before now, and I’m not having it again!”

“What wheel?” said Bets, puzzled. Mr. Goon did one of his snorts and walked off. He couldn’t bear any children, but he particularly detested the Five Find-Outers and Dog. Bets stared after him.

“Well, I didn’t get much out of him,” she thought. “What did he mean about wheels?”

It was lovely when Fatty came back again. He brought Buster with him, of course, and the little Scottie went mad with joy when he saw all his friends.

“He didn’t have too good a time at my grandmother’s.” said Fatty. “There was an enormous ginger cat there that would keep chasing him, and my grandmother insisted on his having a bath every single day. He was awfully miserable really. He would have chased the cat, of course; but he was too much of a gentleman to go after a cat belonging to his hostess.”

“Have you bought any disguises yet?” asked Bets excitedly.

“Just waiting for my birthday,” said Fatty. “It’s tomorrow, as you know. Then, when I’ve got enough money, I’m going up to London to do a spot of shopping.”

“By yourself?” said Larry.

“You bet,” said Fatty. “What grown-up would let me spend my money on disguises? Although we’ve solved two frightfully difficult mysteries, no grown-up would think it was necessary to buy wigs and eyebrows - now would they? Even though at any moment we might have to solve a third mystery.”

Put like that, it seemed a really urgent matter to buy disguises of all sorts. Fatty was so very serious about it. Bets felt that the third mystery might be just round the corner.

“Fatty, can we try out the disguises when you buy them?” she said.

“Of course,” said Fatty. “We’ll have to practise wearing them. It will be fun.”

“Have you brought the invisible ink with you this afternoon?” asked Pip. “That’s what I want to see!”

“Can you see invisible ink?” asked Bets. “I shouldn’t have thought you could.”

The others laughed. “Silly! The ink isn’t invisible - it’s only the writing you do with it that is.”

“I’ve got a bottle,” said Fatty. “It’s very expensive.”

He took a bottle from his pocket. It was quite small, and contained a colourless liquid which, to Bets, looked like water.

Fatty took out his note-book and a pen with a clean new nib. He put the bottle on the table, and undid the screw-top.

“Now I’ll write a secret letter,” he said, “and my writing will be invisible.”

Bets leaned over him to see. She lost her balance and jerked hard against the table. The bottle of invisible ink was jolted over, rolled to the edge of the table, and neatly emptied its contents on the floor in a small round puddle, near Buster.

“Woof!” said Buster in surprise, and began to lick it up. But the taste was horrid. He stopped and looked up at the alarmed children, his pink tongue hanging out.

“Oh, Buster! Buster, you’ve drunk invisible ink!” cried Bets, almost in tears. “Fatty, will he become invisible?”

“No, idiot,” said Fatty. “Well, that’s the end of the ink. What a clumsy you are Bets!”

“I’m terribly, terribly sorry,” said poor Bets. “I just sort of slipped. Oh, Fatty, now we can’t write in invisible writing.”

Daisy mopped up the rest of the ink. All the children were disappointed. Buster still hung out his tongue, and had such a disgusted look on his face that Larry fetched him some water to take the nasty taste out of his mouth.

“Well, I know one or two more ways of writing invisibly,” said Fatty, much to Bets’ relief. “Any one got an orange? Now, watch out for a little magic!”

There was a dish of oranges in the room. Bets fetched them. She watched with great interest as Fatty made a hole in one, and squeezed the yellow juice into a cup.

“There!” he said, “orange or lemon juice makes quite good invisible ink, you know.”

The others didn’t know. They thought Fatty was very clever immediately to think of some more invisible ink when Bets had upset his bottle.

He took a clean sheet of paper, dipped his pen in the orange juice, and wrote what looked like a letter. He said out loud what he was writing, and it made the children giggle:

“DEAR CLEAR-ORF, - I suppose you think you will solve the next mystery first. Well, you won’t. Your brains want oiling a bit. They creak too much. Hugs and kisses from

“THE FIVE FIND-OUTERS AND DOG.”

The children giggled, especially at the last bit. “You are an idiot, Fatty,” said Pip. “It’s a good thing old Clear-Orf won’t get the letter.”

“Oh, we’ll send it all right,” said Fatty, “but as it’s written in invisible ink he won’t be able to read it, poor mutt!”

There was nothing to be seen on the sheet of notepaper. The orange-juice ink was certainly invisible!

“But, Fatty, how can any one read invisible writing?” said Daisy.

“Easy,” said Fatty. “I’ll show you how to read this kind. Got an electric iron anywhere?”

“Yes,” said Pip. “But I don’t expect Mother would let us have it. She seems to think that anything she lends us is bound to get broken. Anyway, whatever do yo want an iron for?”

“Wait and see,” said Fatty. “Haven’t you got an ordinary flat-iron, Pip, if we can’t borrow the electric one? There must be one in the kitchen.”

There was. The cook said Pip might have it. “If you break that, I’d be surprised!” she said, and Pip sped upstairs carrying the heavy old iron.

“Heat it on the fire,” said Fatty. So it was put on the fire, and well heated. When Fatty judged that it was warm enough, he took it off the fire, being careful to hold it with an iron-holder.

“Now watch,” he said, and in excitement they all watched. Fatty ran the iron lightly over the sheet on which he had written his invisible letter.

“There it is! It’s all coming up in faint brown letters!” cried Bets, thrilled. “Look! ‘My dear Clear-Orf -’ ”

“ ‘I suppose you will think...’ ” read Pip, in delight.

“Yes, it’s visible now. Golly, that’s clever, Fatty. I would never have thought that ordinary orange juice could be used as invisible ink!”

“It’s better to know that than to know about the proper invisible ink,” said Larry. “That’s expensive, but you only want an orange for this. It’s marvellous, Fatty. Let’s all write letters.”