“You will write a note to the others to say that you have discovered something wonderful here, and are guarding it, and will they all come to the garden as soon as possible,” he said.

“Oh! - and I suppose you think that you can catch them too when they come, and lock them up till you’ve finished whatever secret business you are on!” said Fatty.

“Exactly,” said the man. “We think it would be better to hold you all prisoner here till we have finished our affairs. Then you can tell what you like.”

“Well, if you think I shall write a letter that will bring my friends into your hands, you’re jolly well mistaken!” said Fatty hotly. “I’m not such a coward as that!”

“Are you not?” said the thin-lipped man, and he looked at Fatty so strangely that the boy trembled. What would this horrible man do to him if he refused to write the note? Fatty didn’t dare to think.

He tried to stare back bravely at the man, but it was difficult. Fatty wished desperately he had not gone into this midnight venture so light-heartedly. He longed for old Buster. But perhaps it was as well that Buster was not there. These men might kick him and misuse him cruelly.

“We shall lock you up,” said the thin-lipped man. “We have to go in a little while, but we shall come back soon. You will write this note whilst we are gone. If it is not done by the time we come back, there will be trouble for you, bad trouble - trouble you will not forget all the rest of your life.”

Fatty’s spirits went up a little when he heard he was to be locked up. He might be able to escape if so! He had a folded newspaper in his pocket. He was sure he could use his trick of getting out of a locked room all right. Then his high spirits sank again.

“We will lock you in this so-comfortable room,” said the red-faced man. “And we will give you paper and pen and ink. You will write a nice, excited note that will bring your friends here quickly. You can throw it out of the window.”

Fatty knew he could never escape from the secret room. A thick carpet ran right to the door. There was no space beneath the edge of the door to slip a key. None at all. He would be a real prisoner. He could not even escape down the tree because the window was so heavily barred.

The thin-lipped man placed a sheet of notepaper on a table, and laid beside it a pen and a little ink-stand.

“There you are,” he said. “You will write this note in your own way and sign it. What is your name?”

“Frederick Trotteville,” said Fatty gloomily.

“You are called Freddie, then, are you not?” said the thin-lipped man. “You will sign your letter ‘Freddie,’ and when your friends come into the garden, I will fling your note from the window - but you will not speak to them.”

The red-faced man looked at his watch. “We must go,” he said. “It is time. Everything is ready here. We will get the rest of these interfering kids and lock them up till we have finished. It won’t hurt them to starve for a day or two in an empty room!”

They went out of the room. Fatty heard the key turn in the lock. He was a prisoner. He stared gloomily at the shut door. It was his own fault that he was in this fix. But he wasn’t going to get the others into it too - no, not even if those men beat him black and blue!

Fatty heard the footsteps of the men clattering down the uncarpeted stairs. He heard the front door close quietly. He heard the sound of a car starting up. The men had gone.

He tried the door. It was locked all right. He went to the window. It was pitch-dark outside. He opened the window and felt the bars. They were too close together for him to slip out between them. He was indeed a prisoner.

He went and sat down again, shivering. Fright and the winter’s chill made him shake all over. He saw the electric fire and decided to put it on. He might as well be warm, anyway!

He sat down once more and gazed gloomily at the sheet of notepaper. What a bad detective he was, to allow himself to be caught like this! It was terribly careless. The others would never admire him again.

“Well, I shan’t write that letter, anyway,” thought the boy, but he trembled to think what his punishment might be if he didn’t.

Then an idea came to him. It was really brilliant. He sat and thought about it for a while. Yes - it would work if only the others were bright enough to catch on to the idea too!

“I’ll write an invisible letter on this sheet of paper, and I’ll write a letter in ink on it as well!” thought Fatty. “I bet Pip and the others will think of testing it for secret writing. Golly - what an idea this is! To write two letters on one sheet, one seen and the other unseen! I bet the men will never think of that!”

He looked at the sheet of paper. It was faintly ruled with lines. He could write his secret letter between the lines and the other letter on the lines! When the others tested it for secret writing, they would then be able to read his real letter easily.

Fatty’s hands shook with excitement. He might be able to do something startling now! He must think carefully what to write. The men who used this room were evil, and they used it as a meeting place for evil reasons. They must be stopped. They were evidently in the middle of some big affiair at the moment, and it was up to Fatty to stop them.

He took a rather squashy orange from his pocket. He looked round for a glass. There was one on the shelf. He squeezed his orange into it, then picked up the pen the men had left. The nib was clean and new.

Should he write the visible letter first, or the secret one? Fatty decided on the visible one, because it would be easier then to write the invisible one, as he could see where he had written the first letter.

He began:

“DEAR FIND-OUTERS - I have made a wonderful discovery, most awfully exciting. I can’t leave here, because I am guarding something - but I want to show you what it is. All of you come as soon as you can, and I will let you in when you knock. - Yours,

‘FREDDIE.’ ”

That seemed all right - just what the man had commanded him to write. But the others would smell a rat as soon as they saw the name “Freddie” at the bottom. He always signed himself Fatty in notes like this.

Then he set to work to write the letter in secret ink - or rather in orange juice.

“DEAR FIND-OUTERS” - he wrote - “Don’t take any notice of the visible letter. I’m a prisoner here. There’s some very dirty work going on; I don’t quite know what. Get hold of Inspector Jenks AT ONCE and tell him everything. He’ll know what to do. Don’t come near the place, any of you. - Yours ever,

‘FATTY.’ ”

That just took him to the bottom of the sheet. Not a trace of the secret writing was visible; only a few sentences of the inked writing were to be seen. Fatty felt pleased. Now, if only the others guessed there was a secret message and read it, things might be all right.

“Inspector Jenks will see to things,” thought Fatty, and it was comforting to think of the clever, powerful Inspector of Police, their very good friend, knowing about this curious affair. Fatty thought of him - his broad cheerful face, his courtesy, his tallness, his shrewdness.

It was now about six o’clock. Fatty yawned. He had had a poor night. He was hungry and tired, but warmer now. He curled himself up on the sofa again and slept.

He was awakened by the men coming into the room again. He sat up, blinking. Daylight now came in through the window.

The thin-lipped man saw the paper on the table and picked it up. He read the letter in silence and then handed it to the other man.

“This is all right,” he said. “We’ll bag all the silly little idiots, and give them a sharp lesson. Will they all come down to see where you are, boy?”