case again.

"But who forced it? Why should they? When? But the door was locked?"
These exclamations burst from us disjointedly.

Poirot answered them categorically--almost mechanically.

"Who? That is the question. Why? Ah, if I only knew. When? Since I was
here an hour ago. As to the door being locked, it is a very ordinary
lock. Probably any other of the doorkeys in this passage would fit it."

We stared at one another blankly. Poirot had walked over to the
mantel-piece. He was outwardly calm, but I noticed his hands, which from
long force of habit were mechanically straightening the spill vases on
the mantel-piece, were shaking violently.

"See here, it was like this," he said at last. "There was something in
that case--some piece of evidence, slight in itself perhaps, but still
enough of a clue to connect the murderer with the crime. It was vital
to him that it should be destroyed before it was discovered and its
significance appreciated. Therefore, he took the risk, the great risk,
of coming in here. Finding the case locked, he was obliged to force it,
thus betraying his presence. For him to take that risk, it must have
been something of great importance."

"But what was it?"

"Ah!" cried Poirot, with a gesture of anger. "That, I do not know! A
document of some kind, without doubt, possibly the scrap of paper Dorcas
saw in her hand yesterday afternoon. And I--" his anger burst forth
freely--"miserable animal that I am! I guessed nothing! I have behaved
like an imbecile! I should never have left that case here. I should
have carried it away with me. Ah, triple pig! And now it is gone. It is
destroyed--but is it destroyed? Is there not yet a chance--we must leave
no stone unturned--"

He rushed like a madman from the room, and I followed him as soon as I
had sufficiently recovered my wits. But, by the time I had reached the
top of the stairs, he was out of sight.

Mary Cavendish was standing where the staircase branched, staring down
into the hall in the direction in which he had disappeared.

"What has happened to your extraordinary little friend, Mr. Hastings? He
has just rushed past me like a mad bull."

"He's rather upset about something," I remarked feebly. I really did not
know how much Poirot would wish me to disclose. As I saw a faint smile
gather on Mrs. Cavendish's expressive mouth, I endeavoured to try and
turn the conversation by saying: "They haven't met yet, have they?"

"Who?"

"Mr. In