The secret agent and his trusty lieutenant were masquerading as natives of Russian Poland, and it may be told that their desperate mission was to enter Warsaw, where the slightest indiscretion or betrayal would put them in graves alongside of that daring spy of Roque's who failed to conceal his identity. It was the midnight hour when Billy was awakened by a man enveloped from neck to foot in a grayish-brown overcoat, from under the head cape of which came the voice of Roque: "Take this" (slipping a fold of coarse paper into the hand of the drowsy lad), "and if you do not hear from me after three days, read what is written, and follow the instructions to the letter. Not a look at the message, remember, for three days; to be exact, the morning of the fourth day. You hear me?" Billy sleepily nodded his head. Out on the turbid tide of the yellow river beyond the German trenches two shrouded figures silently launched a flatboat and drifted away in the darkness. "What's doing?" This was Henri's morning question, preceding a swallow of coffee. "If I knew what was in here I could probably tell you a whole lot that I don't know at present." Billy displayed the closely folded packet containing Roque's instructions. When Henri was advised of the conditions imposed he accepted the trust as a matter of course. It had never been a habit of the boys to break faith. CHAPTER XXI. HELD IN WARSAW. THE din of battle had long since ceased to be an inspiration of terror with the Boy Aviators. They were case-hardened by continual contact with the war game, and too careless, perhaps, of flying lead. Reclining in the trenches, they indulged in all sorts of surmises as to the whereabouts of Roque and Schneider, wagered back and forth, one way and the other, on the proposition of whether the chief would appear in person within the allotted time or put it up to them to interpret the message in Billy's pocket. With the passing of two days, the hours in the next one seemed to move on leaden wings. "I don't even know in what direction to look for his coming," complained Henri. "If he is coming," he corrected himself. "No use getting in a stew about it," advised Billy, concealing the fact that he himself was nearing the boiling point as the last few hours of waiting wore away. The morning of the fourth day, and no sign or sight of the absentees. Billy and Henri sat in council, and the former opened the paper that had haunted his dreams during the previous restless night. "If alive, we are in Warsaw." "I guessed that once." Billy lifted his eyes from the paper. "Go on," impatiently urged Henri.