But his mind was elsewhere.  He was wondering suddenly about Cayley.  Cayley was just an ordinary man—­like himself.  Bill had had little jokes with him sometimes; not that Cayley was much of a hand at joking.  Bill had helped him to sausages, played tennis with him, borrowed his tobacco, lent him a putter .... and here was Antony saying that he was what?  Well, not an ordinary man, anyway.  A man with a secret.  Perhaps a murderer.  No, not a murderer; not Cayley.  That was rot, anyway.  Why, they had played tennis together.

“Now then, Watson,” said Antony suddenly.  “It’s time you said something.”

“I say, Tony, do you really mean it?”

“Mean what?”

“About Cayley.”

“I mean what I said, Bill.  No more.”

“Well, what does it amount to?”

“Simply that Robert Ablett died in the office this afternoon, and that Cayley knows exactly how he died.  That’s all.  It doesn’t follow that Cayley killed him.”

“No.  No, of course it doesn’t.”  Bill gave a sigh of relief.  “He’s just shielding Mark, what?”

“I wonder.”

“Well, isn’t that the simplest explanation?”

“It’s the simplest if you’re a friend of Cayley and want to let him down lightly.  But then I’m not, you see.”

“Why isn’t it simple, anyhow?”

“Well, let’s have the explanation then, and I’ll undertake to give you a simpler one afterwards.  Go on.  Only remember the key is on the outside of the door to start with.”

“Yes; well, I don’t mind that.  Mark goes in to see his brother, and they quarrel and all the rest of it, just as Cayley was saying.  Cayley hears the shot, and in order to give Mark time to get away, locks the door, puts the key in his pocket and pretends that Mark has locked the door, and that he can’t get in.  How’s that?”

“Hopeless, Watson, hopeless.”

“Why?”

“How does Cayley know that it is Mark who has shot Robert, and not the other way round?”

“Oh!” said Bill, rather upset.  “Yes.”  He thought for a moment, “All right.  Say that Cayley has gone into the room first, and seen Robert on the ground.”

“Well?”

“Well, there you are.”

“And what does he say to Mark?  That it’s a fine afternoon; and could he lend him a pocket-handkerchief?  Or does he ask him what’s happened?”

“Well, of course, I suppose he asks what happened,” said Bill reluctantly.

“And what does Mark say?”

“Explains that the revolver went off accidentally during a struggle.”

“Whereupon Cayley shields him by doing what, Bill?  Encouraging him to do the damn silliest thing that any man could possibly do confess his guilt by running away!”

“No, that’s rather hopeless, isn’t it?” Bill thought again.  “Well,” he said reluctantly, “suppose Mark confessed that he’d murdered his brother?”

“That’s better, Bill.  Don’t be afraid of getting away from the accident idea.  Well then, your new theory is this.  Mark confesses to Cayley that he shot Robert on purpose, and Cayley decides, even at the risk of committing perjury, and getting into trouble himself, to help Mark to escape.  Is that right?”