But certainly Elsie’s evidence did not prove anything more than the mere presence of Mark in the room.  “It’s my turn now; you wait.”  That was not an immediate threat;—­it was a threat for the future.  If Mark had shot his brother immediately afterwards it must have been an accident, the result of a struggle, say, provoked by that “nasty-like” tone of voice.  Nobody would say “You wait” to a man who was just going to be shot.  “You wait” meant “You wait, and see what’s going to happen to you later on.”  The owner of the Red House had had enough of his brother’s sponging, his brother’s blackmail; now it was Mark’s turn to get a bit of his own back.  Let Robert just wait a bit, and he would see.  The conversation which Elsie had overheard might have meant something like this.  It couldn’t have meant murder.  Anyway not murder of Robert by Mark.

“It’s a funny business,” thought Antony.  “The one obvious solution is so easy and yet so wrong.  And I’ve got a hundred things in my head, and I can’t fit them together.  And this afternoon will make a hundred and one.  I mustn’t forget this afternoon.”

He found Bill in the hall and proposed a stroll.  Bill was only too ready.  “Where do you want to go?” he asked.

“I don’t mind much.  Show me the park.”

“Righto.”

They walked out together.

“Watson, old man,” said Antony, as soon as they were away from the house, “you really mustn’t talk so loudly indoors.  There was a gentleman outside, just behind you, all the time.”

“Oh, I say,” said Bill, going pink.  “I’m awfully sorry.  So that’s why you were talking such rot.”

“Partly, yes.  And partly because I do feel rather bright this morning.  We’re going to have a busy day.”

“Are we really?  What are we going to do?”

“They’re going to drag the pond—­beg its pardon, the lake.  Where is the lake?”

“We’re on the way to it now, if you’d like to see it.”

“We may as well look at it.  Do you haunt the lake much in the ordinary way?”

“Oh, no, rather not.  There’s nothing to do there.”

“You can’t bathe?”

“Well, I shouldn’t care to.  Too dirty.”

“I see ....  This is the way we came yesterday, isn’t it?  The way to the village?”

“Yes.  We go off a bit to the right directly.  What are they dragging it for?”

“Mark.”

“Oh, rot,” said Bill uneasily.  He was silent for a little, and then, forgetting his uncomfortable thoughts in his sudden remembrance of the exciting times they were having, said eagerly, “I say, when are we going to look for that passage?”

“We can’t do very much while Cayley’s in the house.”

“What about this afternoon when they’re dragging the pond?  He’s sure to be there.”

Antony shook his head.

“There’s something I must do this afternoon,” he said.  “Of course we might have time for both.”