The superintendent touched his bell.  Two minutes later John Jones was brought into the room.

“Amy!” exclaimed he.  “How came you here?”

He ran up to her, and they greeted each other affectionately.  The woman, who had controlled herself up to this point, burst into tears.  Jones turned in wrath toward Nick.

“Haven’t we had enough of this infernal nonsense?” he exclaimed.  “You have raised the devil with my business and scared my wife into a fit.  Now let me out, and arrest the Ameer of Afghanistan.  He had more to do with this affair than I did.”

Nick did not reply, but he made a secret sign to the superintendent.

“You are at liberty, Mr. Jones,” said Byrnes, calmly.  “I regret that it was necessary to detain you so long.”

“I have no complaint to make against you,” said Jones.  “It was that man’s work, and he shall pay for it.”

He scowled at Nick, and then, after bowing to the superintendent, walked out of the room with his wife on his arm.

“Shall I call a man?” asked Byrnes.

“If you please,” said Nick.  “My force is pretty busy.”

“Musgrave!” said the superintendent.

A man appeared so suddenly that he seemed to come out of the wall.

“Shadow the couple that has just left here,” said Byrnes.  “You are under Mr. Carter’s orders until dismissed by him.”

Musgrave turned to Nick.

“I have no special instructions,” said Nick, “except that you keep your eyes on the woman.”

The officer saluted, and vanished almost as quickly as he had come in.

At half-past seven o’clock that evening Musgrave was on guard outside the flat, the address of which had been given to Nick by Jones.

An old man selling papers came along the street, calling “Extra!” in a cracked voice.

Musgrave bought a paper.

“Well,” said the newsman, in Nick Carter’s voice, “what have you to report?”

“From headquarters they went to an employment agency on Sixth avenue.  They engaged a colored girl as a servant.

“They then came straight here, and the girl followed them.  Mr. and Mrs. Jones have not been out since.”

“Are you sure of that?”

“Perfectly.  There is no way to get out of that house from the rear.”

“How about the fire-escape?”

“There is only that one on the side which you can see.  The little yard back of the house is walled in by buildings.”

“So Mr. and Mrs. Jones must be inside?”

“Yes.”

“And the girl?”

“She is out.  She has been going on errands half a dozen times, but usually to the grocer’s or the butcher’s around the corner.  I don’t know where she has gone this time.  She’s been out about a quarter of an hour.”

“All right.  I’m going over there.”

Nick changed his disguise to that in which Jones had seen him.  He did it in the hall of the flat house, while waiting for the door to be opened in answer to his ring.