Upon this scene came Ling Chu, imperturbable, expressionless, bringing with him his own atmosphere of mystery.

"Well," said Tarling, "what have you discovered?" and even Whiteside checked his enthusiasm to listen.

"Two people came up the stairs last night," said Ling Chu, "also the master." He looked at Tarling, and the latter nodded. "Your feet are clear," he said; "also the feet of the small-piece woman; also the naked feet."

"The naked feet?" said Tarling, and Ling Chu assented.

"What was the naked foot--man or woman?" asked Whiteside.

"It may have been man or woman," replied the Chinaman, "but the feet were cut and were bleeding. There is mark of blood on the gravel outside."

"Nonsense!" said Whiteside sharply.

"Let him go on," warned Tarling.

"A woman came in and went out----" continued Ling Chu.

"That was Miss Rider," said Tarling.

"Then a woman and a man came; then the bare-footed one came, because the blood is over the first women's footmarks."

"How do you know which was the first woman and which was the second?" asked Whiteside, interested in spite of himself.

"The first woman's foot was wet," said Ling Chu.

"But there had been no rain," said the detective in triumph.

"She was standing on the grass," said Ling Chu, and Tarling nodded his head, remembering that the girl had stood on the grass in the shadow of the bushes, watching his adventure with Milburgh.

"But there is one thing I do not understand master," said Ling Chu. "There is the mark of another woman's foot which I cannot find on the stair in the hall. This woman walked all round the house; I think she walked round twice; and then she walked into the garden and through the trees."

Tarling stared at him.

"Miss Rider came straight from the house on to the road," he said, "and into Hertford after me."

"There is the mark of a woman who has walked round the house," insisted Ling Chu, "and, therefore, I think it was a woman whose feet were bare."

"Are there any marks of a man beside us three?"

"I was coming to that," said Ling Chu. "There is a very faint trace of a man who came early, because the wet footsteps are over his; also he left, but there is no sign of him on the gravel, only the mark of a wheel-track."

"That was Milburgh," said Tarling.

"If a foot has not touched the ground," explained Ling Chu, "it would leave little trace. That is why the woman's foot about the house is so hard for me, for I cannot find it on the stair. Yet I know it came from the house because I can see it leading from the door. Come, master, I will show you."

He led the way down the stairs into the garden, and then for the first time Whiteside noticed that the Chinaman was bare-footed.

"You haven't mixed your own footmarks up with somebody else's?" he asked jocularly.

Ling Chu shook his head.

"I left my shoes outside the door because it is easier for me to work so," he said calmly, slipping his feet into his small shoes.

He led the way to the side of the house, and there pointed out the footprints. They were unmistakably feminine. Where the heel was, was a deep crescent-shaped hole, which recurred at intervals all round the house. Curiously enough, they were to be found in front of almost every window, as though the mysterious visitor had walked over the garden border as if seeking to find an entrance.

"They look more like slippers than shoes to me. They're undoubtedly a woman's," said Whiteside, examining one of the impressions. "What do you think, Tarling?"

Tarling nodded and led the way back to the room.

"What is your theory, Ling Chu?" he asked.

"Somebody came into the house," said the Chinaman, "squeezed through the door below and up the stairs. First that somebody killed and then went to search the house, but could not get through the door."

"That's right," said Whiteside. "You mean the door that shuts off this little wing from the rest of the house. That was locked, was it not, Tarling, when you made the discovery?"

"Yes," said Tarling, "it was locked."

"When they found they could not get into the house," Ling Chu went on, "they tried to get through one of the windows."

"They, they?" said Tarling impatiently. "Who are they? Do you mean the woman?"

The new theory was disturbing. He had pierced the second actor in the tragedy--a brown vitriol burn on the back of his hand reminded him of his existence--but who was the third?

"I mean the woman," replied Ling Chu quietly.

"But who in God's name wanted to get into the house after murdering Mrs. Rider?" asked Whiteside irritably. "Your theory is against all reason, Ling Chu. When a person has committed a murder they want to put as much distance between themselves and the scene of the crime as they can in the shortest possible space of time."

Ling Chu did not reply.

"How many people are concerned in this murder?" said Tarling. "A bare-footed man or woman came in and killed Mrs. Rider; a second person made the round of the house, trying to get in through one of the windows----"

"Whether it was one person or two I cannot tell," replied Ling Chu.

Tarling made a further inspection of the little wing. It was, as Ling Chu had said and as he had explained to the Chinaman, cut off from the rest of the house, and had evidently been arranged to give Mr. Milburgh the necessary privacy upon his visits to Hertford. The wing consisted of three rooms; a bedroom, leading from the sitting-room, evidently used by Mrs. Rider, for her clothes were hanging in the wardrobe; the sitting-room in which the murder was committed, and the spare room through which he had passed with Odette to the gallery over the hall.

It was through the door in this room that admission was secured to the house.

"There's nothing to be done but to leave the local police in charge and get back to London," said Tarling when the inspection was concluded.

"And arrest Milburgh," suggested Whiteside. "Do you accept Ling Chu's theory?"

Tarling shook his head.

"I am loath to reject it," he said, "because he is the most amazingly clever tracker. He can trace footmarks which are absolutely invisible to the eye, and he has a bushman's instinct which in the old days in China led to some extraordinary results."

They returned to town by car, Ling Chu riding beside the chauffeur, smoking an ............