And Antony didn’t hear.  Indeed, he had played up to Cayley splendidly.  Not only had he called attention to the open window, but he had carefully explained to Cayley why Mark had chosen this particular window in preference to the office window.  And Cayley had agreed that probably that was the reason.  How he must have chuckled to himself!  But he was still a little afraid.  Afraid that Antony would examine the shrubbery.  Why?  Obviously because there was no trace of anyone having broken through the shrubbery.  No doubt Cayley had provided the necessary traces since, and had helped the Inspector to find them.  Had he even gone as far as footmarks in Mark’s shoes?  But the ground was very hard.  Perhaps footmarks were not necessary.  Antony smiled as he thought of the big Cayley trying to squeeze into the dapper little Mark’s shoes.  Cayley must have been glad that footmarks were not necessary.

No, the open window was enough; the open window and a broken twig or two.  But quietly, quietly.  Antony mustn’t hear.  And Antony had not heard ....  But he had seen a shadow on the wall.

They were outside on the lawn again now, Bill and Antony, and Bill was listening open-mouthed to his friend’s theory of yesterday’s happenings.  It fitted in, it explained things, but it did not get them any further.  It only gave them another mystery to solve.

“What’s that?” said Antony.

“Mark.  Where’s Mark?  If he never went into the office at all, then where is he now?”

“I don’t say that he never went into the office.  In fact, he must have gone.  Elsie heard him.”  He stopped and repeated slowly, “She heard him, at least she says she did.  But if he was there, he came out again by the door.”

“Well, but where does that lead you?”

“Where it led Mark.  The passage.”

“Do you mean that he’s been hiding there all the time?” Antony was silent until Bill had repeated his question, and then with an effort he came out of his thoughts and answered him.

“I don’t know.  But look here.  Here is a possible explanation.  I don’t know if it is the right one I don’t know, Bill; I’m rather frightened.  Frightened of what may have happened, of what may be going to happen.  However, here is an explanation.  See if you can find any fault with it.”

With his legs stretched out and his hands deep in his pockets, he lay back on the garden-seat, looking up to the blue summer sky above him, and just as if he saw up there the events of yesterday being enacted over again, he described them slowly to Bill as they happened.

“We’ll begin at the moment when Mark shoots Robert.  Call it an accident; probably it was.  Mark would say it was, anyhow.  He is in a panic, naturally.  But he doesn’t lock the door and run away.  For one thing, the key is on the outside of the door; for another, he is not, quite such a fool as that.  But he is in a horrible position.  He is known to be on bad terms with his brother; he has just uttered some foolish threat to him, which may possibly have been overheard.  What is he to do?  He does the natural thing, the thing which Mark would always do in such circumstances.  He consults Cayley, the invaluable, inevitable Cayley.