“Has Cayley got to be out of the house for the other thing too?”

“Well, I think he ought to be.”

“I say, is it anything rather exciting?”

“I don’t know.  It might be rather interesting.  I daresay I could do it at some other time, but I rather fancy it at three o’clock, somehow.  I’ve been specially keeping it back for then.”

“I say, what fun!  You do want me, don’t you?”

“Of course I do.  Only, Bill don’t talk about things inside the house, unless I begin.  There’s a good Watson.”

“I won’t.  I swear I won’t.”

They had come to the pond—­Mark’s lake—­and they walked silently round it.  When they had made the circle, Antony sat down on the grass, and relit his pipe.  Bill followed his example.

“Well, Mark isn’t there,” said Antony.

“No,” said Bill.  “At least, I don’t quite see why you know he isn’t.”

“It isn’t ‘knowing,’ it’s ‘guessing,’” said Antony rapidly.  “It’s much easier to shoot yourself than to drown yourself, and if Mark had wanted to shoot himself in the water, with some idea of not letting the body be found, he’d have put big stones in his pockets, and the only big stones are near the water’s edge, and they would have left marks, and they haven’t, and therefore he didn’t, and oh, bother the pond; that can wait till this afternoon.  Bill, where does the secret passage begin?”

“Well, that’s what we’ve got to find out, isn’t it?”

“Yes.  You see, my idea is this.”

He explained his reasons for thinking that the secret of the passage was concerned in some way with the secret of Robert’s death, and went on: 

“My theory is that Mark discovered the passage about a year ago the time when he began to get keen on croquet.  The passage came out into the floor of the shed, and probably it was Cayley’s idea to put a croquet-box over the trap-door, so as to hide it more completely.  You know, when once you’ve discovered a secret yourself, it always seems as if it must be so obvious to everybody else.  I can imagine that Mark loved having this little secret all to himself and to Cayley, of course, but Cayley wouldn’t count and they must have had great fun fixing it up, and making it more difficult for other people to find out.  Well then, when Miss Norris was going to dress-up, Cayley gave it away.  Probably he told her that she could never get down to the bowling-green without being discovered, and then perhaps showed that he knew there was one way in which she could do it, and she wormed the secret out of him somehow.”

“But this was two or three days before Robert turned up.”

“Exactly.  I am not suggesting that there was anything sinister about the passage in the first place.  It was just a little private bit of romance and adventure for Mark, three days ago.  He didn’t even know that Robert was coming.  But somehow the passage has been used since, in connection with Robert.  Perhaps Mark escaped that way; perhaps he’s hiding there now.  And if so, then the only person who could give him away was Miss Norris.  And she of course would only do it innocently not knowing that the passage had anything to do with it.”