“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: