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