Coates returned with the Rover, just as the body of the victim was being removed, and his account of what had occurred was simple enough, and followed the lines which we had anticipated.  He had locked up and then gone to the garage for the car as I had directed him to do, returning to the cottage in time to admit Eric Coverly, whom he showed into the study, having informed him that I should be back in less than ten minutes.  He had then proceeded to Denmark Hill railway station only to find, as I had found, that the appointment was a hoax and “the man with a box” a myth.

“You see,” said Gatton, “the scheme of the plotter was simply this:  to get Coates out of the way for a long enough time to allow the substitution of the telephone to be accomplished.  The fact that Coates had closed the windows before leaving the house didn’t interfere very much with the scheme.  It’s an old-fashioned catch on the ante-room window, and I have seen the marks upon the brass-work where it was forced from the outside with the blade of a knife.  For the person who opened the window to take out the real telephone and put the other in its place was easy; and all that remained was to lift the gas-cylinder on to the shed and partly reclose the window as we found it.  Coates, even if he had troubled to look, would not have noticed any difference in the dusk.  It is the next move, however, which I find most interesting.”

Gatton spoke with repressed excitement, and: 

“What do you mean by ’the next move’?” I asked.

“Well,” he replied, “we have good evidence to show that the assassin possesses an almost Napoleonic capacity for working by the time-table.  Witness the employment of Constable Bolton in the Red House affair—­which showed that our man was perfectly acquainted with the movements of the officer on that beat and timed his scheme accordingly.  Very well ... having laid the telephone trap in your ante-room—­did our man hurry away and make the call in person, which brought Coverly to the ’phone?—­or did he remain watching the house and give the signal to some one else to do it?”

“I cannot imagine, Gatton.  Nor does the point strike me as important.”

“No?” said Gatton, smiling triumphantly.  “Then I must explain.  Whereas, in the Red House, the scheme worked automatically—­for the time of Sir Marcus’s arrival was fixed—­in the present instance, some one had to watch for your return from the mythical appointment!”

“For my return?”

“Unquestionably!  This scheme was arranged for your benefit, Mr. Addison.  Unknowingly, poor Coverly saved you from a dreadful fate at the price of his own life!  You see, they did not know that Coverly was coming here!  Now, it will not have escaped your attention that he wore a soft felt hat, a light overcoat, and carried a black cane.  So did you when you went out to keep the appointment made by the assassin!”