In order to avoid arousing Mostyn Brown's suspicions that his house was being watched, we had given up, apparently, all observation on the place during daylight. But not in reality. In a house on the other side of the Square, directly facing that occupied by Mostyn Brown, I had hired a room on the third floor, and from the window, with the help of powerful field-glasses, we could keep the house under the strictest watch. We had not even to enter the Square to reach our tower of vantage, for there was a back entrance from an adjoining street. Towards this eyrie I had bent my steps, and on arriving I found Aubert in a state of suppressed excitement. "Look!" he said, handing me the glasses, and, taking them from him, I levelled them at Mostyn Brown's room. The ribbon on the parrot's cage had been changed to yellow! But this was not all. The sun shone full on the window of Mostyn Brown's house and his room was strongly illuminated. The field-glasses showed us that Mostyn Brown was at home, a most unusual thing in the day-time, and that with him was Blind Heinrich. How Heinrich had got there we could not imagine. Aubert had not seen him enter. They were seated on chairs drawn up to the table, and were poring intently over a book, apparently making memoranda on sheets of paper. As we watched, Madame Gabrielle, habited as a coster girl and carrying a huge basket of flowers, came slowly along the Square, past Mostyn Brown's house and round past the house in which we were seated. I saw her flutter a signal, and, with her arm resting naturally on the side of the basket, she rapidly tapped out a message with her nimble fingers. "Heinrich has been with Mostyn Brown for the past two hours," she spelt out. "He came straight from Hereford Road and went into the next house from the back." Evidently there was some way of communication at the rear of the two houses. I had now no time to waste, and, leaving Aubert and Madame Gabrielle to keep the necessary watch, I hurried off to Whitehall, where I was soon in deep talk with the astute and enterprising chief of the London defences, a keen officer who by sheer merit had forced himself to the very front rank in aircraft service. "Good!" he said, when I had told him my news. "I think we shall give them a surprise to-night. Perhaps you would like to see how we work. Sit down for a bit." And he turned to his big table, on which stood a telephone.