Not often does one see a gray-painted landaulet; hence, the odd though hardly remarkable fact occurred to Theydon that a precisely similar gray automobile had occupied the center of the station yard at Waterloo when he took a taxi from the rank.

Admittedly he was in a nervous and excited state.  It could hardly be otherwise after the strain of that astounding conversation with Forbes, and there was no prospect of the tension being relaxed until the close of the interview with the detectives, which he now regarded as the worse ordeal of the two.

But this subconscious neurasthenia in no wise affected the reflex action of his ordinary faculties.  When, on leaving the square, and while his cab was rattling along an aristocratic thoroughfare leading to Knightsbridge, he peered through a tiny observation window in the back of the vehicle, and ascertained that the gray car was stealing along quietly about a hundred yards in the rear, he began to believe that its presence both at Waterloo and outside Mr. Forbes’s residence could not be wholly accidental.  When he had watched its persistent treading on his heels along Piccadilly its intent became almost unmistakable.

The route to Innesmore Mansions traversed some of London’s main arteries, but, despite the rush of traffic due to the first flight of homewardbound playgoers, the gray car kept steadily on his track.  Amused at first, be became angry because of a notion which grew out of the wonderment of finding himself the object of this persistent espionage.

To make sure, and at the same time discover the sort of person who was spying on him, he adopted a ruse.  Leaning out, when about to cross Oxford Street into Tottenham Court Road, he said to his driver:  “Turn sharp to the right in Store Street, and pull up.  I’ll tell you when to go on again.”

The man obeyed.  Theydon posted himself at the outer window, and in a space of time so short that the excellence of the gray car’s accelerator was amply demonstrated, the pursuer swung into sight.  A stolid-faced chauffeur at the wheel did not appear discomfited at coming on his quarry thus unexpectedly.  He whirled past, seemingly quite oblivious of Theydon’s fixed stare.  Though the weather was mild he wore an overcoat with upturned collar, so that between its protecting flaps and a low-peaked cap his face was well hidden.  Still, Theydon received an impression of a curiously wooden physiognomy.

The man might have been an automaton for all the heed he gave to the taxi or its inquisitive occupant.  But his aspect was almost forgotten in the far stranger discovery that the car was empty.  Both windows were open, and the bright lights of a corner shop flashed into the interior, yet not a soul was visible.  Moreover, the car sped on unhesitatingly, stopping some two hundred yards ahead.

So far as Theydon could tell, no one alighted.  He jotted down the number—­ XY 1314—­ on his shirt cuff.