Between Berkeley Square and Scotland Yard, Fairfield consumed ten cigarettes in sharp jerky puffs. Yet he was scarcely conscious of lighting one. Indeed, as he climbed the wide flight of steps at the main entrance, it seemed as though no palpable interval of time had elapsed since he had been practically turned out of her father's house by Eileen Meredith.

Heldon Foyle put away the bundle of documents that contained the history of the case as the baronet was announced, and waved his visitor to a chair.

"Well?" he asked.

Fairfield shrugged his shoulders. "A nice mess you've got me into," he complained. "Why didn't you tell me you knew the secret of the cipher?"

The detective's face was full of ingenuous surprise as he answered—

"Didn't I? I thought I made it perfectly clear to you. I am sorry that you misunderstood. I should have made it plainer. What has gone wrong?"

Sir Ralph made an impatient gesture. "Oh, what's the use of talking nonsense? You did not tell me that you knew the cipher, and as a consequence Lady Eileen now knows that you know."

The superintendent gave no indication of the chagrin with which the news filled him. His features were perfectly expressionless. A part of his plans had failed from excess of caution. He did not need Fairfield to tell him what had happened. He could make a fairly accurate guess as to the manner in which he had been unwittingly betrayed. His thoughts turned at once to the question of what the girl would do. If he had judged her right, she would try to warn Grell. Either she knew his address or not, but it was unlikely that she did, as they were communicating in cipher. The obvious thing for her to do was to try to stop the advertisement. There was, however, little he could do. He had men on duty in Berkeley Square and in Fleet Street. He would soon hear of any new developments.

"That's a pity," he said reflectively. "It may mean a re-arrangement of our plans. And believe me, Sir Ralph, I badly regret now that I did not go into fuller details with you. What happened?"

Stumblingly, Sir Ralph recapitulated the scene at Berkeley Square, giving even the epithets by which the girl had addressed him. Foyle tapped lightly on his desk with the end of a penholder. The event had been as he thought. He looked Sir Ralph straight in the eye.

"She told you that you were a spy—that I had used you as a tool," he said sharply. "You have been hurt by her words. I don't want you to feel that you are anything but a free agent, or to do anything that you consider dishonourable. But I must know whether you are still willing to act with us, or whether you wish to stand aside."

Fairfield threw the stump of cigarette viciously into the fire.

"I am acting with you, of course," he answered sullenly, "though I wish you to ask for my help only when it is absolutely necessary. What I complain of is, that I have not been frankly treated, and that I have been placed in an invidious position with Lady Eileen. You must remember that I have feelings, and that it is not pleasant to be told one is acting as a spy, especially by—by an old friend. You know, Mr. Foyle, that I have only been wishful to serve those I have known."

There was something pathetic in his endeavour to justify his actions to himself. Foyle murmured a sympathetic, "I understand—yes, yes, I know," and then became thoughtful.

"After all," he said at last, "this does not make us so very badly off. You are openly on our side now, Sir Ralph, so there can be no fear of your again being accused of acting in an underhand manner. There is nothing more to be done at the moment. I will keep you posted as to any steps we are taking."

"Very well. Good morning, Mr. Foyle."

The baronet was gone. The superintendent resumed his perusal of documents. He felt some little compunction at what had happened. Yet it was his business to clear up the mystery, and to use what instruments came to his hand, so long as the law was not violated. There is a code of etiquette in detective work in which the first and most important rule is: "Take advantage of every chance of bringing a criminal to justice." In using Fairfield as an instrument, Foyle was merely following that code.

In a little, Foyle had finished and sent for Green. The chief inspector came with a report.

"A woman brought the advertisement to Fleet Street, sir," he said. "Blake has just telephoned up that he and Lambert are keeping her under observation. He 'phoned earlier that Lady Eileen Meredith had been there."

"Yes, I suppose so. What does the advertisement say?"

"He couldn't tell me on the 'phone. He had to hurry away to look after the woman. It is being sent up by taxicab."

"That's good. By the way, Green, keep half-a-dozen men handy, and be about yourself."

"Very good, sir. Is there anything on?"

"I don't quite know. We may have to go out in a hurry. I'll tell you after we have deciphered the advertisement."