“Which other theory?”

“That Mark had killed Robert accidentally and had gone to Cayley for help, and that Cayley, having hidden him in the passage, locked the office door from the outside and hammered on it.”

“Yes, but you were so dashed mysterious about that.  I asked you what the point of it was, and you wouldn’t say anything.”  He thought for a little, and then went on, “I suppose you meant that Cayley deliberately betrayed Mark, and tried to make him look like a murderer?”

“I wanted to warn you that we should probably find Mark in the passage, alive or dead.”

“And now you don’t think so?”

“Now I think that his dead body is there.”

“Meaning that Cayley went down and killed him afterwards after you had come, after the police had come?”

“Well, that’s what I shrink from, Bill.  It’s so horribly cold-blooded.  Cayley may be capable of it, but I hate to think of it.”

“But, dash it all, your other way is cold-blooded enough.  According to you, he goes up to the office and deliberately shoots a man with whom he has no quarrel, whom he hasn’t seen for fifteen years!”

“Yes, but to save his own neck.  That makes a difference.  My theory is that he quarrelled violently with Mark over the girl, and killed him in sudden passion.  Anything that happened after that would be self-defense.  I don’t mean that I excuse it, but that I understand it.  And I think that Mark’s dead body is in the passage now, and has been there since, say, half-past two yesterday afternoon.  And to-night Cayley is going to hide it in the pond.”

Bill pulled at the moss on the ground beside him, threw away a handful or two, and said slowly, “You may be right, but it’s all guess-work, you know.”

Antony laughed.

“Good Lord, of course it is,” he said.  “And to-night we shall know if it’s a good guess or a bad one.”

Bill brightened up suddenly.

“To-night,” he said.  “I say, to-night’s going to be rather fun.  How do we work it?”

Antony was silent for a little.

“Of course,” he said at last, “we ought to inform the police, so that they can come here and watch the pond to-night.”

“Of course,” grinned Bill.

“But I think that perhaps it is a little early to put our theories before them.”

“I think perhaps it is,” said Bill solemnly.

Antony looked up at him with a sudden smile.

“Bill, you old bounder.”

“Well, dash it, it’s our show.  I don’t see why we shouldn’t get our little bit of fun out of it.”

“Neither do I. All right, then, we’ll do without the police to-night.”

“We shall miss them,” said Bill sadly, “but ’tis better so.”

There were two problems in front of them:  first, the problem of getting out of the house without being discovered by Cayley, and secondly, the problem of recovering whatever it was which Cayley dropped into the pond that night.