He thought he had heard a low moan, and catching the eye of Whiteside, he saw that the Scotland Yard man had detected the same sound.
"Sounds like somebody groaning," he said.

"Listen!" He bent his head and waited, and presently it came again.
In two strides Tarling was at the door of Ling Chu's sleeping place, but it was locked.

He stooped to the key-hole and listened, and again heard the moan.

With a thrust of his shoulder he had broken the door open and dashed in.
The sight that met his eyes was a remarkable one.

There was a man lying on the bed, stripped to the waist.