I looked at the envelope, carrying it to the light. I noticed that the stamp and the postmark had been torn off and inwardly I praised the young man for his discretion. The flap was stuck down, though it had of course been cut at the top where the letter was taken out, and to all intents it had not been tampered with. I examined it with a magnifying glass, however, and saw that suspicious gum line which can always be seen in a letter that has been opened and surreptitiously closed. “This envelope has been steamed,” I said, “the letter has been read, replaced, fastened down by a kind of spirit gum which the censors use, and smoothed with a hot iron.” “How do you know?” he asked in alarm. “I have opened too many myself,” said I, with a smile, “not to recognize the signs.” “But supposing they had brought up the secret writing,” he said, “they could not understand the code.” I thought for a moment and presently I said: “There is only one person in the world who can read that code and that person is a woman. More than that, Herr Lieutenant,” I added proudly, “that woman is the dearest friend I have in the world, Miss Kathleen O’Mara of Chicago, U.S.A.” And I told him something of this delightful girl’s history, of how her father had been a Fenian and how she was bitterly anti-English. She had taken up codes and ciphers as a hobby, and she had come to be so expert that you were always reading in the Chicago papers articles either written by her or about her. There was not a cryptograph that ever appeared in the agony column of a London newspaper that she couldn’t discover. “You said her name was O’Mara,” said the Herr Lieutenant thoughtfully; “is she a tall, slim girl, with dark hair and blue eyes?” It was my turn to be amazed. “Herr Lieutenant,” I said, “even your illustrious father could not have described her more accurately.” “And is her husband a tallish man with bristling black eyebrows and fierce black moustache?” I drew myself up stiffly. “The lady’s husband, Herr Lieutenant, you see before you in prospect. She is unmarried.” He looked at me and shook his head. “Well, then, it is not the same lady. This was a Mrs. Hagan, the wife of Captain Hagan of the United States Secret Service.” I stepped back and clasped my brows. Now I remembered the man! Hagan from Washington! And she had married him. By heavens! When I think of the depth of woman’s duplicity I could despise the race. She, the Irish patriot, the strafer of England, to send that man to me in the hope that I should commit myself! Thou perfidious betrayer of one who entertained for thee naught but the tenderest, holiest feelings! Oh, what a low woman! “I see it all now, Herr Lieutenant,” I said, “this woman was brought down to your camp to unravel your cipher. When did you receive it?” He thought a moment. “The night she left.”