During the previous ten days Department Z. had learned a great deal about the comings and goings of Mr. Montagu Naylor of Streatham. It had become manifest to Sage that he had to do with a man who had reduced cunning and caution to a fine art. His every act seemed to have been carefully thought out beforehand, not only in relation to himself, but to what might grow directly out of it. During a walk he would sometimes turn suddenly and proceed swiftly in the direction from which he had come, as if he had forgotten something, looking keenly at every one he passed. At others he would step into a shop, where he could be seen keeping a careful watch through the window. A favourite trick was to walk briskly round a corner, then stop and look in some shop window with a small mirror held in the palm of his hand. From the first Malcolm Sage had realised that the conventional methods of shadowing a suspect would be useless for his purpose. Those in whom Department Z. were interested would be old hands at the game, and to set a single person to watch them would inevitably result in the discovery of what was afoot. He therefore set at least three men, or women, to dog the footsteps of the suspect. These would follow each other at intervals of from twenty-five to a hundred yards, according to the district in which they were operating. At a signal that the first in the line was dropping out, the trail would be taken up by number two, who in turn would relinquish the work to number three. Sometimes as many as six were allocated to one shadowing. This method had the additional advantage of enabling the Department to assure itself that the watchers were not in turn being watched. It was no uncommon thing for a suspect to arrange to have himself shadowed in order to ascertain whether or no there were any one on his track. This was a favourite device with Mr. Naylor. For nearly two years Department Z. had been endeavouring to solve the problem of a secret organisation, with the offshots of which they were constantly coming into contact. The method this organisation adopted was one of concentration upon a single object. At one time it would be at the sailing of vessels from home ports, at another the munitions output, or again the anti-aircraft defences of London. Malcolm Sage was convinced that somewhere there was at work a controlling mind, one that weighed every risk and was prepared for all eventualities. Individuals had been shadowed, some had been arrested, much to Sage's disgust. The efforts of the organisation had frequently been countered and its objects defeated; but Department Z. had hitherto been unable to penetrate beyond the outer fringe. The most remarkable thing of all was that no document of any description had been discovered, either on the person of those arrested, or through the medium of the post.