Lupin was standing with the paper in his hand; and I was able at my ease to watch, on his youthful features, that extraordinary mobility of expression which baffles all observers and constitutes his great strength and his chief safeguard. By what signs can one hope to identify a face which changes at pleasure, even without the help of make-up, and whose every transient expression seems to be the final, definite expression?... By what signs? There was one which I knew well, an invariable sign: Two little crossed wrinkles that marked his forehead whenever he made a powerful effort of concentration. And I saw it at that moment, saw the tiny tell-tale cross, plainly and deeply scored. He put down the sheet of paper and muttered: "Child's play!" The clock struck half-past five. "What!" I cried. "Have you succeeded?... In twelve minutes?..." He took a few steps up and down the room, lit a cigarette and said: "You might ring up Baron Repstein, if you don't mind, and tell him I shall be with him at ten o'clock this evening." "Baron Repstein?" I asked. "The husband of the famous baroness?" "Yes." "Are you serious?" "Quite serious." Feeling absolutely at a loss, but incapable of resisting him, I opened the telephone-directory and unhooked the receiver. But, at that moment, Lupin stopped me with a peremptory gesture and said, with his eyes on the paper, which he had taken up again: "No, don't say anything.... It's no use letting him know.... There's something more urgent ... a queer thing that puzzles me.... Why on earth wasn't the last sentence finished? Why is the sentence...." He snatched up his hat and stick: "Let's be off. If I'm not mistaken, this is a business that requires immediate solution; and I don't believe I _am_ mistaken." He put his arm through mine, as we went down the stairs, and said: