The professor was illuminated by the orange light from the fireplace, his back to the room where all the guests had now assembled. He was staring at something which he had previously placed on the mantle, and presently he picked it up and held it tightly in his right hand. He had not changed clothes, and so he still wore the tweed jacket that had been torn and muddied in the chase along the river bank.

The young soldier Theodore Pendleton; Mr. Price the butler and his wife Agatha; Lady Boethia Pendleton, wife of the deceased; Sidney Prescott the shipping millionaire; Oscar Wainwright, accompanied by Helen Clewes, the mysterious woman in purple whose presence at once thrilled and terrified the professor; the aged Doctor, and of course, swaggering Colin O’Rourke. They were all there. Any of them could have done it, but only one of them had. The door closed softly.

“Really, Professor Laslow, I don’t see the use of you calling us all here to talk about father’s murder. The police already have Sir Humphrey in custody!”

He turned sharply to face young Theodore, concealing his right hand in his pocket.

“That is precisely why I have gathered you all here, Captain Pendleton! I intend to prove that my friend Sir Humphrey could not possibly have committed the crime!”

“This is rubbish!” shouted Boethia Pendleton. Her dog looked up briefly, then sank back down into the comfort of the prodigious lap in which it had been sleeping.

The doctor looked around at the room nervously, then took a step forward. “I agree, my dear. Professor, this is all highly unusual. I examined the Commodore’s body myself, and I can assure you that the bullet which killed him was fired from Sir Humphrey’s revolver. I am convinced that—”

”—How can you be so convinced, doctor, about matters of which you are so totally ignorant! You are a physician, and well-versed, I am certain, in the finer points of constipation and influenza, but have you also studied criminology? Forensics? Deduction? Then perhaps you will keep silent!”

“See here, Laslow, you’ve no right to talk to him that way!” Erupted Sidney Prescott, who was seated by the door.

“Aww, let ‘im talk, he’s doon no harm! What ha’ye to hide, sare, ah woonder?”

“Thank you Mr. O’Rourke,” the professor continued kindly. “Now, what do we know about the attack on Commodore Pendleton? We know it was done with a service revolver that belonged to Sir Humphrey, but that could easily have been stolen when our rooms were ransacked this afternoon.”

“But the door to father’s room was locked,” said Theodore, “and the key was found in Sir Humphrey’s pocket!”

“A simple matter! The key was planted on Sir Humphrey sometime during dinner. Sleight of hand, nothing more!”

“But if the key was planted during dinner, how would the killer still have it to break into the Commodore’s bedroom at midnight,” asked Oscar Wainwright.

“How indeed,” smiled the professor. “You see, the key was merely a diversion to throw attention away from the killer’s real modus operandi. He entered the commodore’s bedroom not via the door, but the dumbwaiter! He then crept across the room in the moonless dark and fired Sir Humphrey’s revolver at the sleeping Commodore, and exited the way he came!”

“Balderdash! What man could fit inside the dumbwaiter?”

With a dramatic flourish, the professor pulled his hand from his coat pocket and opened it to the lapping firelight, revealing a small, articulated wooden nub with a piece of twine secured to one end and hanging loosely down. All the guests looked on, most with curiousity, one with dread.

“Not a man, lady Pendleton! Not a man, but a monkey! Man’s old ancestor, returned from the jungle to carry out a sinister revenge!”

The professor wore a cold expression on his face, and his eyes narrowed into angry slits. “I suspected it from the beginning, but one thing confounded me: how could a monkey, lacking opposeable thumbs, and therefore—as science has proven—incapable of pulling a trigger, manage to shoot a revolver accurately enough to kill a man?”

“Then I discovered this. An ingenious device, fashioned by expert craftsmen to resemble a small thumblike digit, and tied to the monkey’s palm to give him the grip necessary to perform the fell deed! It must have slipped off when he made good his escape, and lay there until I found it just moments ago.”

“But why would it do such a thing?” asked Mr. Price timidly, “the Master was beloved throughout the animal kingdom, it’s well known.”

“Among animals, perhaps, but there was at least one human who hated him enough to pervert the nature of an otherwise peaceful creature, and through brutal psychological manipulation, teach him to become a thief and a murderer: first to steal a pistol, and then to use it remorselessly!”

Suddenly the professor wheeled around to face the perpetrator, his finger outstretched and pointing like the very arm of the law. “Someone like you, Helen Clewes!”