Mrs. Hilton put her head into the room again to see how the children were getting on with their new friend. She was simply horrified to see him apparently in floods of tears.

“What’s the matter?” she said. “What have you been doing to the boy?”

“Nothing,” said the children indignantly. “I only just asked him about his bad leg,” added Bets.

The boy gave a loud howl, limped across the room to the door, pushed by the distressed Mrs. Hilton, and disappeared down the stairs. “Ah, ma jambe, ma jambe!” he wailed as he went.

“What’s jambe?” asked Bets, bewildered.

“Leg. He’s yelling out, ‘Oh, my leg, my leg!’ ” said Pip. “He’s mad, I think.”

“I must ring up Mrs. Trotteville and ask her about the boy,” said Mrs. Hilton. “Poor child - he doesn’t seem at all well. I wish I hadn’t brought him up to you now. He did seem very tongue-tied and shy, I must say.”

The front door crashed shut. The children crowded to the window and watched the extraordinary French boy go limping down the drive. He still had his handkerchief in his hand, which every now and again he dabbed at his eyes.

“Well, if that’s one of Fatty’s friends I’m glad he didn’t ask us to play with him,” said Larry in disgust.

“I’ll just leave the boy time to get back to Mrs. Trotteville’s,” said Mrs. Hilton, “and then I really must telephone her to ask if he’s arrived all right and to apologize for your upsetting him so.”

The children stared at her indignantly.

“Upsetting him!” said Pip. “We didn’t do anything of the sort, He’s potty.”

“Don’t use that silly word about people,” said Mrs. Hilton.

“Well, dippy then,” said Pip, and got a glare from his mother. She was very particular about the way Pip and Bets spoke and acted.

“I’m sorry to think that you couldn’t put a little foreign boy like that at his ease,” she said, and spent a few more minutes saying the same kind of thing. Then she went to the telephone to ring up Mrs. Trotteville.

But she apparently got on to Fatty, who politely informed Mrs. Hilton that his mother was out and could he take any message for her?

“Well, no, not exactly,” said Mrs. Hilton. “It’s only that I’m rather worried about a friend of yours, Frederick, who called here with a note just now. I took him up to be with the others for a few minutes, and when I went in later something had happened to make him very upset. He fled from the house, weeping bitterly. I just wondered if he had come back all right.”

“Yes, he’s back,” said Fatty cheerfully. “He came and told me how nice the others had been to him, and what fun he had had. He said could he come to tea with them this afternoon, he would so enjoy it.”

Mrs. Hilton was extremely surprised to hear all this. She didn’t say anything for a moment, then she turned to the listening children.

“Er - the boy seems to have got back all right, and to have recovered,” she said. “He wants to come to tea with you this afternoon.”

There was an astonished and horrified silence. Nobody wanted the boy.

“Mother, we can’t have him!” said Pip, in an agonized whisper. “He’s awful; he really is. Do say we’re all going up to Larry’s to tea. Larry, can we come? We simply can’t have that awful boy here again.”

Larry nodded. Mercifully Mrs. Hilton seemed to agree with them, and she turned to the telephone again.

“Oh, Frederick, are you there? Will you tell your friend that Pip and Bets are going out to tea with Larry and Daisy this afternoon, so they won’t be able to have your little French friend. I’m so sorry.”

“Good for you, Mother!” said Pip, when she put the telephone down. “Golly, wouldn’t it have been simply awful to have that boy stuck here for hours. I bet old Fatty wanted us to have him to tea just to get rid of him. I bet the boy didn’t really ask to come. He was scared stiff of us all.”

“Well, you’d better come up to us this afternoon,” said Daisy, “seeing that we’ve told Fatty that. Come up as soon after dinner as you can - about half-past two, if you like.”

“Right,” said Pip. “We’ll be along. Golly, how can Fatty put up with friends like that?”

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!”