“Exactly, sir,” Jupiter agreed.

Mr. Hitchcock frowned. “However, all that is most circumstantial, young man. It fits Professor Walsh, but it also fits others at the ranch. What was it that made you narrow your deductions to Walsh?”

“The pistol he carried, sir,” Jupiter said triumphantly.

“The pistol?” the director repeated, staring at Bob’s notes. “I see nothing here about the pistol — nothing special, that is.”

“No, sir, not the weapon itself, but the way he held it,” Jupiter said quickly. “You see, the El Diablo who captured us held the pistol in his left hand. He wore his holster on his left hip. But all the books and pictures indicated that El Diablo was right-handed, when we found the skeleton of the real El Diablo in the cave it had the pistol in its right hand, too. So — ”

“Thunderation!” Mr. Hitchcock exclaimed. “How could I have overlooked that! Of course, young Jones, only the professor had the theory that El Diablo was left-handed! He was tripped up by his own pet theory!”

“Yes, sir, he was,” Jupiter said with a grin. “You see, he really was a professor, too, as well as a thief. As Mr. Reston said, he had spent five years establishing another identity. He really was Professor Walsh, and he really was an expert on California history. He was actually writing a book about El Diablo, and when he impersonated the bandit he automatically conformed to his own theory and acted left-handed!”

Mr. Hitchcock laughed uproariously. “You boys have done more than well, young Jones. This may be your most ingeniously deduced case, and I will be glad to introduce it. His left hand, by thunder!”

The boys all beamed happily at this high praise. Then Jupiter held out the old pistol that he and Pete had found in the hands of the real El Diablo.

“We thought, sir, that you might like to have this as a memento of The Mystery of the Moaning Cave,” he said.

“Ah, the pistol of the real El Diablo.” Mr. Hitchcock looked at the old gun with awe. “I most certainly will treasure it. Actually, you young sleuths did more than explain the moaning and solve the diamond robbery. You finally provided an ending for the legend of El Diablo.”

“Golly!” Pete exclaimed. “We did, didn’t we?”

“There remains only one problem,” Mr. Hitchcock said, and his eyes twinkled. “Is there actually an Old One in that pool in the cave? Might it have killed El Diablo?”

Jupiter became thoughtful again. He stared into the distance.

“Well, sir, the legend of The Old One has been passed down for a long time. Possibly there is a true basis for it. It might be interesting to return to the cave and see if there is anything in that pool.”

“Oh, no!” groaned Bob and Pete together.

But all Jupiter said then was, “Hmmmmm… I wonder.”

The three boys filed out of the office, and Alfred Hitchcock looked at the pistol on his desk and smiled. Once again The Three Investigators had explained a mystery that had baffled adults. He wondered what riddle would involve them next. Perhaps it would be The Old One in the cave.

If not, it would be something equally mysterious, he was sure.