“You bet!” said Fatty, and slipped them into his mouth. At once his whole appearance changed as he grinned round, the frightful sticking-out teeth making him look completely unlike himself.

Fatty looked fine when the Find-Outers at last left him, taking Buster with them. Fatty had decided that it wouldn’t do to leave the little dog behind in the house as he might bark all night long. So he was to spend the night with Larry and Daisy. Bets wanted him, but Pip said that their mother would be sure to ask all kinds of why and wherefore questions if Buster suddenly appeared for the night, and that might lead to something awkward.

So Larry took him home, and Buster, rather surprised, trotted along with him and Daisy, limping every now and again whenever he remembered. He quite thought that Fatty would be along to fetch him from Larry’s sooner or later.

Fatty sat up fairly late reading. He was in his French-boy disguise, and looked fine. If the maid had popped her head into his room she would get a shock. But nobody saw him at all.

At about ten o’clock Fatty slipped out of the house. The moon was almost full, and shone brightly down on the white snow. Fatty’s footsteps made no sound at all.

He went down the road, took the way over the hill, and at last walked down Chestnut Lane, keeping well to the hedge, in the black shadows there. He saw nobody. Mr. Goon was not about that night, being busy nursing a very bad cold which had suddenly and most annoyingly seized him. Otherwise he had fully meant to hang about Milton House to see if he could find out anything that night.

Now he was in bed sneezing hard and dosing himself with hot lemon and honey, determined to get rid of the cold by the next day, in case those tiresome children got ahead of him in this new mystery.

So there was no one to watch Fatty. He slipped in at the drive gate, kept to the shadows, and made his way round the house, hoping that no one would notice his footprints the next day. He came to the little tumble-down summerhouse and went in. He had two thick rugs with him, and put them down on the seat.

He had a look up at the secret room, with its strange bars. Was there any one there yet? Would any one come that night?

It was cold. Fatty went back to the summer-house and cuddled himself up in the rugs. He soon felt warm again. He grew rather sleepy, and kept blinking to keep himself awake. He heard the church clock in the village strike eleven. Then he must have fallen asleep, for the next thing he knew was the clock striking again! This time it struck twelve.

“Golly!” said Fatty, “midnight! I must have fallen asleep. Well - as nothing has happened, and no one has come, or is likely to come as late as this, I’ll just pop down the coal-hole!”

Fatty had put on his oldest clothes. His mother was not as particular as Pip’s, but even she would remark on clothes marked with coal-dust. Fatty looked a proper little ruffian as he threw off the rugs and stood listening in the moonlight. He had on the curly wig, he had made his face very pale, he had stuck on dark eyebrows, and, of course, he had the awful teeth. He was certainly enough to startle any one if there had been someone to see him.

He made his way round the hedges of the garden to the kitchen entrance, keeping well in the shadows. He came to the coal-hole. Snow had covered it again, but Fatty knew just about where it was. He cleared the snow away from it, and bent down to pull up the round iron lid.

It needed a jolly good tug, but at last up it came, unexpectedly suddenly, so that Fatty sat down with a bump, and the lid clanged down, making quite a noise.

Fatty held his breath, but nothing happened. He got up cautiously, pushed the lid to one side, and then shone his torch down the dark opening to see how far below the floor was.

Fortunately for him there was a heap of coal just below the hole. He could let himself down on it fairly easily. So down he went, and landed on the coal, which at once gave beneath him, so that he went slithering down the side of the heap.

He picked himself up and switched on his torch. He saw a flight of stone steps leading upwards to a shut door - the kitchen or scullery door, he guessed. He went up slowly, and turned the handle of the door.

It opened into a large scullery, into which the moon shone brightly. It was completely empty. He went into the next room, which was a kitchen. That, too, was empty, but in the dust of the floor Fatty saw the same large footprints that he had seen in the snowy drive the day before.

“Perhaps I can see into the secret room!” thought the boy, his heart beating fast. It was a queer feeling to be all alone in a deserted house, knowing that people came there secretly for some mysterious reasons.

Fatty felt certain there was nobody at all in the house, but all the same he jumped at any moving shadow, and almost leapt out of his skin when a floor-board creaked loudly under his foot.

He looked into room after room. All were completely empty. He explored all the ground floor, the first floor, and the second floor. The secret room was on the third floor, at the top of the house. Fatty went up the stairs to the last floor, trying to walk as quietly as possible even though he felt so certain that there was nobody else in the house but himself.

He came to the top floor. He looked into the first room he came to. It was empty. He looked into the next one, that was empty too. But the third one was the secret room!

Fatty pushed open the door quietly and slowly. He peeped in. It lay silent and still in the brilliant moonlight - a very comfortable room, large, high-ceilinged like all the rooms, and very well furnished.

Fatty walked round the room. It had evidently been roughly cleaned and thoroughly dusted not long before. A little pile of tins of meat and fruit stood on a shelf. The kettle on the stove had water in it. A tin of tea was on the table. Books stood on the window-sill, and Fatty turned over the pages of some. They were in a foreign language and he couldn’t understand a word.

The sofa had been prepared as a kind of bed, for the cushions were piled at one end, and cosy rugs had been folded there. It was all very strange.

“I suppose I’d better get back to the summer-house,” thought Fatty. “I wish I could find some letters or documents of some sort that would tell me a bit about this queer room. But there don’t seem to be any.”

He sat down on the sofa and yawned. Then his eye caught sight of a small cupboard in the wall He wondered what was in it. He got up - but the cupboard was locked. Fatty put his hand into his pocket and brought out a perfectly extraordinary collection of keys. He had secretly been making a board of these, as he had leant that most detectives can lock or unlock doors of cupboards. They had queer keys called skeleton keys which could apparently unlock with ease almost anything that needed a key.

But a skeleton key had proved impossible to buy, and, indeed, had led to many awkward questions being put by the shopkeepers whom he had asked for one. So Fatty had been forced to collect any old key which he could find, and he now had a very varied collection which weighed down the pocket of his coat considerably. He took them all out.

Most patiently and methodically Fatty tried first one key and then another in the lock of the little cupboard, and to his delight, and also his surprise, one key did manage to unlock the door!

Inside was a small book, a kind of notebook, and entered in it were numbers and names, nothing else at all. It seemed very dull to Fatty.

“Perhaps Inspector Jenks may like to have a look at it,” he thought, and he pocketed the little book and locked the cupboard door again. “We shall soon be reporting this mystery to him, and he may like to have all the bits of evidence we can find.”