“Who?”

“Cayley, of course.”  Very gravely and slowly he expounded.  “He came in here in order to open the window.  He shut the door so that I shouldn’t hear him open the window.  He opened the window.  I came in here and found the window open.  I said, ’This window is open.  My amazing powers of analysis tell me that the murderer must have escaped by this window.’  ‘Oh,’ said Cayley, raising his eyebrows.  ‘Well,’ said he, ‘I suppose you must be right.’  Said I proudly, ‘I am.  For the window is open,’ I said.  Oh, you incomparable ass!”

He understood now.  It explained so much that had been puzzling him.

He tried to put himself in Cayley’s place—­Cayley, when Antony had first discovered him, hammering at the door and crying, “Let me in!” Whatever had happened inside the office, whoever had killed Robert, Cayley knew all about it, and knew that Mark was not inside, and had not escaped by the window.  But it was necessary to Cayley’s plans—­to Mark’s plans if they were acting in concert—­that he should be thought so to have escaped.  At some time, then, while he was hammering (the key in his pocket) at the locked door, he must suddenly have remembered—­with what a shock!—­that a mistake had been made.  A window had not been left open!

Probably it would just have been a horrible doubt at first.  Was the office window open?  Surely it was open!  Was it? ....  Would he have time now to unlock the door, slip in, open the French windows and slip out again?  No.  At any moment the servants might come.  It was too risky.  Fatal, if he were discovered.  But servants were stupid.  He could get the windows safely open while they were crowding round the body.  They wouldn’t notice.  He could do it somehow.

And then Antony’s sudden appearance!  Here was a complication.  And Antony suggesting that they should try the window!  Why, the window was just what he wanted to avoid.  No wonder he had seemed dazed at first.

Ah, and here at last was the explanation why they had gone the longest way round and yet run.  It was Cayley’s only chance of getting a start on Antony, of getting to the windows first, of working them open somehow before Antony caught him up.  Even if that were impossible, he must get there first, just to make sure.  Perhaps they were open.  He must get away from Antony and see.  And if they were shut, hopelessly shut, then he must have a moment to himself, a moment in which to think of some other plan, and avoid the ruin which seemed so suddenly to be threatening.

So he had run.  But Antony had kept up with him.  They had broken in the window together, and gone into the office.  But Cayley was not done yet.  There was the dressing-room window!  But quietly, quietly.  Antony mustn’t hear.